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SQL

  • The SQL SELECT Statement
  • The SELECT DISTINCT statement
  • The WHERE clause
  • The AND & OR operators
  • The ORDER BY keyword
  • The SQL INSERT INTO Statement
  • The UPDATE statement
  • The DELETE statement
  • An SQL Injection
  • The SQL SELECT TOP Clause
  • SQL LIKE Operator
  • A wildcard character
  • SQL IN operator
  • The BETWEEN operator
  • SQL aliases
  • SQL joins
  • SQL INNER JOIN
  • The LEFT JOIN
  • SQL RIGHT JOIN Syntax
  • SQL FULL OUTER JOIN Keyword
  • SQL UNION operator
  • SQL SELECT INTO statement
  • SQL INSERT INTO SELECT statement
  • The SQL CREATE DATABASE Statement
  • The SQL CREATE TABLE Statement
  • SQL Constraints
  • SQL NOT NULL Constraint
  • SQL UNIQUE Constraint
  • SQL PRIMARY KEY Constraint
  • SQL FOREIGN KEY Constraint
  • SQL CHECK Constraint
  • SQL DEFAULT Constraint
  • The CREATE INDEX statement
  • SQL DROP INDEX, DROP TABLE, and DROP DATABASE
  • The ALTER TABLE Statement
  • SQL AUTO INCREMENT Field
  • SQL Views
  • SQL Dates
  • SQL NULL Value
  • SQL General Data Types
  • SQL COUNT()

.

The SQL SELECT Statement

The SELECT statement is used to select data from a database.

The result is stored in a result table, called the result-set.

SQL SELECT Syntax
SELECT column_name,column_name
FROM table_name;
and

SELECT * FROM table_name;

The SELECT DISTINCT statement

The SELECT DISTINCT statement is used to return only distinct (different) values.

The SQL SELECT DISTINCT Statement
In a table, a column may contain many duplicate values; and sometimes you only want to list the different (distinct) values.

The DISTINCT keyword can be used to return only distinct (different) values.

SQL SELECT DISTINCT Syntax
SELECT DISTINCT column_name,column_name
FROM table_name;

The WHERE clause

The WHERE clause is used to filter records.

The SQL WHERE Clause
The WHERE clause is used to extract only those records that fulfill a specified criterion.

SQL WHERE Syntax
SELECT column_name,column_name
FROM table_name
WHERE column_name operator value;

The AND & OR operators

The AND & OR operators are used to filter records based on more than one condition.

The SQL AND & OR Operators
The AND operator displays a record if both the first condition AND the second condition are true.

The OR operator displays a record if either the first condition OR the second condition is true.

The ORDER BY keyword

The ORDER BY keyword is used to sort the result-set.

The SQL ORDER BY Keyword
The ORDER BY keyword is used to sort the result-set by one or more columns.

The ORDER BY keyword sorts the records in ascending order by default. To sort the records in a descending order, you can use the DESC keyword.

SQL ORDER BY Syntax
SELECT column_name, column_name
FROM table_name
ORDER BY column_name ASC|DESC, column_name ASC|DESC;

The SQL INSERT INTO Statement

The SQL INSERT INTO Statement
The INSERT INTO statement is used to insert new records in a table.

SQL INSERT INTO Syntax
It is possible to write the INSERT INTO statement in two forms.

The first form does not specify the column names where the data will be inserted, only their values:

INSERT INTO table_name
VALUES (value1,value2,value3,…);
The second form specifies both the column names and the values to be inserted:

INSERT INTO table_name (column1,column2,column3,…)
VALUES (value1,value2,value3,…);

The UPDATE statement

The UPDATE statement is used to update records in a table.

The SQL UPDATE Statement
The UPDATE statement is used to update existing records in a table.

SQL UPDATE Syntax
UPDATE table_name
SET column1=value1,column2=value2,…
WHERE some_column=some_value;
Note Notice the WHERE clause in the SQL UPDATE statement!
The WHERE clause specifies which record or records that should be updated. If you omit the WHERE clause, all records will be updated!

The DELETE statement

The DELETE statement is used to delete records in a table.

The SQL DELETE Statement
The DELETE statement is used to delete rows in a table.

SQL DELETE Syntax
DELETE FROM table_name
WHERE some_column=some_value;
Note Notice the WHERE clause in the SQL DELETE statement!
The WHERE clause specifies which record or records that should be deleted. If you omit the WHERE clause, all records will be deleted!

An SQL Injection

An SQL Injection can destroy your database.

SQL in Web Pages
In the previous chapters, you have learned to retrieve (and update) database data, using SQL.

When SQL is used to display data on a web page, it is common to let web users input their own search values.

Since SQL statements are text only, it is easy, with a little piece of computer code, to dynamically change SQL statements to provide the user with selected data:

Server Code
txtUserId = getRequestString(“UserId”);
txtSQL = “SELECT * FROM Users WHERE UserId = ” + txtUserId;
The example above, creates a select statement by adding a variable (txtUserId) to a select string. The variable is fetched from the user input (Request) to the page.

The rest of this chapter describes the potential dangers of using user input in SQL statements.

SQL Injection
SQL injection is a technique where malicious users can inject SQL commands into an SQL statement, via web page input.

Injected SQL commands can alter SQL statement and compromise the security of a web application.

SQL Injection Based on 1=1 is Always True
Look at the example above, one more time.

Let’s say that the original purpose of the code was to create an SQL statement to select a user with a given user id.

If there is nothing to prevent a user from entering “wrong” input, the user can enter some “smart” input like this:

UserId:

105 or 1=1

Server Result
SELECT * FROM Users WHERE UserId = 105 or 1=1
The SQL above is valid. It will return all rows from the table Users, since WHERE 1=1 is always true.

Does the example above seem dangerous? What if the Users table contains names and passwords?

The SQL statement above is much the same as this:

SELECT UserId, Name, Password FROM Users WHERE UserId = 105 or 1=1
A smart hacker might get access to all the user names and passwords in a database by simply inserting 105 or 1=1 into the input box.

SQL Injection Based on “”=”” is Always True
Here is a common construction, used to verify user login to a web site:

User Name:
Password:
Server Code
uName = getRequestString(“UserName”);
uPass = getRequestString(“UserPass”);

sql = “SELECT * FROM Users WHERE Name =’” + uName + “‘ AND Pass =’” + uPass + “‘”
A smart hacker might get access to user names and passwords in a database by simply inserting ” or “”=” into the user name or password text box.

The code at the server will create a valid SQL statement like this:

Result
SELECT * FROM Users WHERE Name =”” or “”=”” AND Pass =”” or “”=””
The result SQL is valid. It will return all rows from the table Users, since WHERE “”=”” is always true.

SQL Injection Based on Batched SQL Statements
Most databases support batched SQL statement, separated by semicolon.

Example
SELECT * FROM Users; DROP TABLE Suppliers
The SQL above will return all rows in the Users table, and then delete the table called Suppliers.

If we had the following server code:

Server Code
txtUserId = getRequestString(“UserId”);
txtSQL = “SELECT * FROM Users WHERE UserId = ” + txtUserId;
And the following input:

User id:

105; DROP TABLE Suppliers

The code at the server would create a valid SQL statement like this:

Result
SELECT * FROM Users WHERE UserId = 105; DROP TABLE Suppliers
Parameters for Protection
Some web developers use a “blacklist” of words or characters to search for in SQL input, to prevent SQL injection attacks.

This is not a very good idea. Many of these words (like delete or drop) and characters (like semicolons and quotation marks), are used in common language, and should be allowed in many types of input.

(In fact it should be perfectly legal to input an SQL statement in a database field.)

The only proven way to protect a web site from SQL injection attacks, is to use SQL parameters.

SQL parameters are values that are added to an SQL query at execution time, in a controlled manner.

ASP.NET Razor Example
txtUserId = getRequestString(“UserId”);
txtSQL = “SELECT * FROM Users WHERE UserId = @0”;
db.Execute(txtSQL,txtUserId);
Note that parameters are represented in the SQL statement by a @ marker.

The SQL engine checks each parameter to ensure that it is correct for its column and are treated literally, and not as part of the SQL to be executed.

Another Example
txtNam = getRequestString(“CustomerName”);
txtAdd = getRequestString(“Address”);
txtCit = getRequestString(“City”);
txtSQL = “INSERT INTO Customers (CustomerName,Address,City) Values(@0,@1,@2)”;
db.Execute(txtSQL,txtNam,txtAdd,txtCit);
Note
You have just learned to avoid SQL injection. One of the top website vulnerabilities.

Examples
The following examples shows how to build parameterized queries in some common web languages.

SELECT STATEMENT IN ASP.NET:

txtUserId = getRequestString(“UserId”);
sql = “SELECT * FROM Customers WHERE CustomerId = @0”;
command = new SqlCommand(sql);
command.Parameters.AddWithValue(“@0”,txtUserID);
command.ExecuteReader();
INSERT INTO STATEMENT IN ASP.NET:

txtNam = getRequestString(“CustomerName”);
txtAdd = getRequestString(“Address”);
txtCit = getRequestString(“City”);
txtSQL = “INSERT INTO Customers (CustomerName,Address,City) Values(@0,@1,@2)”;
command = new SqlCommand(txtSQL);
command.Parameters.AddWithValue(“@0”,txtNam);
command.Parameters.AddWithValue(“@1”,txtAdd);
command.Parameters.AddWithValue(“@2”,txtCit);
command.ExecuteNonQuery();
INSERT INTO STATEMENT IN PHP:

$stmt = $dbh->prepare(“INSERT INTO Customers (CustomerName,Address,City)
VALUES (:nam, :add, :cit)”);
$stmt->bindParam(‘:nam’, $txtNam);
$stmt->bindParam(‘:add’, $txtAdd);
$stmt->bindParam(‘:cit’, $txtCit);
$stmt->execute();

The SQL SELECT TOP Clause

The SQL SELECT TOP Clause
The SELECT TOP clause is used to specify the number of records to return.

The SELECT TOP clause can be very useful on large tables with thousands of records. Returning a large number of records can impact on performance.

Note: Not all database systems support the SELECT TOP clause.

SQL Server / MS Access Syntax
SELECT TOP number|percent column_name(s)
FROM table_name;

SQL LIKE Operator

The LIKE operator is used in a WHERE clause to search for a specified pattern in a column.

The SQL LIKE Operator
The LIKE operator is used to search for a specified pattern in a column.

SQL LIKE Syntax
SELECT column_name(s)
FROM table_name
WHERE column_name LIKE pattern;

A wildcard character

A wildcard character can be used to substitute for any other character(s) in a string.

SQL Wildcard Characters
In SQL, wildcard characters are used with the SQL LIKE operator.

SQL wildcards are used to search for data within a table.

With SQL, the wildcards are:

Wildcard Description
% A substitute for zero or more characters
_ A substitute for a single character
[charlist] Sets and ranges of characters to match
[^charlist]
or
[!charlist] Matches only a character NOT specified within the brackets

SQL IN operator

The IN Operator
The IN operator allows you to specify multiple values in a WHERE clause.

SQL IN Syntax
SELECT column_name(s)
FROM table_name
WHERE column_name IN (value1,value2,…);

The BETWEEN operator

The BETWEEN operator is used to select values within a range.

The SQL BETWEEN Operator
The BETWEEN operator selects values within a range. The values can be numbers, text, or dates.

SQL BETWEEN Syntax
SELECT column_name(s)
FROM table_name
WHERE column_name BETWEEN value1 AND value2;

SQL aliases

SQL aliases are used to temporarily rename a table or a column heading.

SQL Aliases
SQL aliases are used to give a database table, or a column in a table, a temporary name.

Basically aliases are created to make column names more readable.

SQL Alias Syntax for Columns
SELECT column_name AS alias_name
FROM table_name;
SQL Alias Syntax for Tables
SELECT column_name(s)
FROM table_name AS alias_name;

SQL joins

SQL joins are used to combine rows from two or more tables.

SQL JOIN
An SQL JOIN clause is used to combine rows from two or more tables, based on a common field between them.

The most common type of join is: SQL INNER JOIN (simple join). An SQL INNER JOIN returns all rows from multiple tables where the join condition is met.

Let’s look at a selection from the “Orders” table:

OrderID CustomerID OrderDate
10308 2 1996-09-18
10309 37 1996-09-19
10310 77 1996-09-20

Then, have a look at a selection from the “Customers” table:
CustomerID CustomerName ContactName Country
1 Alfreds Futterkiste Maria Anders Germany
2 Ana Trujillo Emparedados y helados Ana Trujillo Mexico
3 Antonio Moreno Taquería Antonio Moreno Mexico

Notice that the “CustomerID” column in the “Orders” table refers to the “CustomerID” in the “Customers” table. The relationship between the two tables above is the “CustomerID” column.

Then, if we run the following SQL statement (that contains an INNER JOIN):

Example
SELECT Orders.OrderID, Customers.CustomerName, Orders.OrderDate
FROM Orders
INNER JOIN Customers
ON Orders.CustomerID=Customers.CustomerID

SQL INNER JOIN

The INNER JOIN keyword selects all rows from both tables as long as there is a match between the columns in both tables.

SQL INNER JOIN Syntax
SELECT column_name(s)
FROM table1
INNER JOIN table2
ON table1.column_name=table2.column_name;
or:

SELECT column_name(s)
FROM table1
JOIN table2
ON table1.column_name=table2.column_name;
PS! INNER JOIN is the same as JOIN.

The LEFT JOIN

The LEFT JOIN keyword returns all rows from the left table (table1), with the matching rows in the right table (table2). The result is NULL in the right side when there is no match.

SQL LEFT JOIN Syntax
SELECT column_name(s)
FROM table1
LEFT JOIN table2
ON table1.column_name=table2.column_name;
or:

SELECT column_name(s)
FROM table1
LEFT OUTER JOIN table2
ON table1.column_name=table2.column_name;
PS! In some databases LEFT JOIN is called LEFT OUTER JOIN.

SQL RIGHT JOIN Syntax

The RIGHT JOIN keyword returns all rows from the right table (table2), with the matching rows in the left table (table1). The result is NULL in the left side when there is no match.

SQL RIGHT JOIN Syntax
SELECT column_name(s)
FROM table1
RIGHT JOIN table2
ON table1.column_name=table2.column_name;
or:

SELECT column_name(s)
FROM table1
RIGHT OUTER JOIN table2
ON table1.column_name=table2.column_name;
PS! In some databases RIGHT JOIN is called RIGHT OUTER JOIN.

SQL FULL OUTER JOIN Keyword

The FULL OUTER JOIN keyword returns all rows from the left table (table1) and from the right table (table2).

The FULL OUTER JOIN keyword combines the result of both LEFT and RIGHT joins.

SQL FULL OUTER JOIN Syntax
SELECT column_name(s)
FROM table1
FULL OUTER JOIN table2
ON table1.column_name=table2.column_name;

SQL UNION operator

The SQL UNION operator combines the result of two or more SELECT statements.

The SQL UNION Operator
The UNION operator is used to combine the result-set of two or more SELECT statements.

Notice that each SELECT statement within the UNION must have the same number of columns. The columns must also have similar data types. Also, the columns in each SELECT statement must be in the same order.

SQL UNION Syntax
SELECT column_name(s) FROM table1
UNION
SELECT column_name(s) FROM table2;
Note: The UNION operator selects only distinct values by default. To allow duplicate values, use the ALL keyword with UNION.

SQL UNION ALL Syntax
SELECT column_name(s) FROM table1
UNION ALL
SELECT column_name(s) FROM table2;
PS: The column names in the result-set of a UNION are usually equal to the column names in the first SELECT statement in the UNION.

SQL SELECT INTO statement

With SQL, you can copy information from one table into another.

The SELECT INTO statement copies data from one table and inserts it into a new table.

The SQL SELECT INTO Statement
The SELECT INTO statement selects data from one table and inserts it into a new table.

SQL SELECT INTO Syntax
We can copy all columns into the new table:

SELECT *
INTO newtable [IN externaldb]
FROM table1;
Or we can copy only the columns we want into the new table:

SELECT column_name(s)
INTO newtable [IN externaldb]
FROM table1;
The new table will be created with the column-names and types as defined in the SELECT statement. You can apply new names using the AS clause.

SQL INSERT INTO SELECT statement

With SQL, you can copy information from one table into another.

The INSERT INTO SELECT statement copies data from one table and inserts it into an existing table.

The SQL INSERT INTO SELECT Statement
The INSERT INTO SELECT statement selects data from one table and inserts it into an existing table. Any existing rows in the target table are unaffected.

SQL INSERT INTO SELECT Syntax
We can copy all columns from one table to another, existing table:

INSERT INTO table2
SELECT * FROM table1;
Or we can copy only the columns we want to into another, existing table:

INSERT INTO table2
(column_name(s))
SELECT column_name(s)
FROM table1;

The SQL CREATE DATABASE Statement

The CREATE DATABASE statement is used to create a database.

SQL CREATE DATABASE Syntax
CREATE DATABASE dbname;
SQL CREATE DATABASE Example
The following SQL statement creates a database called “my_db”:

CREATE DATABASE my_db;
Database tables can be added with the CREATE TABLE statement.

The SQL CREATE TABLE Statement

The CREATE TABLE statement is used to create a table in a database.

Tables are organized into rows and columns; and each table must have a name.

SQL CREATE TABLE Syntax
CREATE TABLE table_name
(
column_name1 data_type(size),
column_name2 data_type(size),
column_name3 data_type(size),
….
);
The column_name parameters specify the names of the columns of the table.

The data_type parameter specifies what type of data the column can hold (e.g. varchar, integer, decimal, date, etc.).

The size parameter specifies the maximum length of the column of the table.

Tip: For an overview of the data types available in MS Access, MySQL, and SQL Server, go to our complete Data Types Reference.

SQL Constraints

SQL constraints are used to specify rules for the data in a table.

If there is any violation between the constraint and the data action, the action is aborted by the constraint.

Constraints can be specified when the table is created (inside the CREATE TABLE statement) or after the table is created (inside the ALTER TABLE statement).

SQL CREATE TABLE + CONSTRAINT Syntax
CREATE TABLE table_name
(
column_name1 data_type(size) constraint_name,
column_name2 data_type(size) constraint_name,
column_name3 data_type(size) constraint_name,
….
);
In SQL, we have the following constraints:

NOT NULL – Indicates that a column cannot store NULL value
UNIQUE – Ensures that each row for a column must have a unique value
PRIMARY KEY – A combination of a NOT NULL and UNIQUE. Ensures that a column (or combination of two or more columns) have a unique identity which helps to find a particular record in a table more easily and quickly
FOREIGN KEY – Ensure the referential integrity of the data in one table to match values in another table
CHECK – Ensures that the value in a column meets a specific condition
DEFAULT – Specifies a default value for a column
The next chapters will describe each constraint in detail.

SQL NOT NULL Constraint

By default, a table column can hold NULL values.

SQL NOT NULL Constraint
The NOT NULL constraint enforces a column to NOT accept NULL values.

The NOT NULL constraint enforces a field to always contain a value. This means that you cannot insert a new record, or update a record without adding a value to this field.

The following SQL enforces the “P_Id” column and the “LastName” column to not accept NULL values:

Example
CREATE TABLE PersonsNotNull
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255)
)

SQL UNIQUE Constraint

The UNIQUE constraint uniquely identifies each record in a database table.

The UNIQUE and PRIMARY KEY constraints both provide a guarantee for uniqueness for a column or set of columns.

A PRIMARY KEY constraint automatically has a UNIQUE constraint defined on it.

Note that you can have many UNIQUE constraints per table, but only one PRIMARY KEY constraint per table.

SQL UNIQUE Constraint on CREATE TABLE
The following SQL creates a UNIQUE constraint on the “P_Id” column when the “Persons” table is created:

SQL Server / Oracle / MS Access:

CREATE TABLE Persons
(
P_Id int NOT NULL UNIQUE,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255)
)
MySQL:

CREATE TABLE Persons
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
UNIQUE (P_Id)
)
To allow naming of a UNIQUE constraint, and for defining a UNIQUE constraint on multiple columns, use the following SQL syntax:

MySQL / SQL Server / Oracle / MS Access:

CREATE TABLE Persons
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
CONSTRAINT uc_PersonID UNIQUE (P_Id,LastName)
)
SQL UNIQUE Constraint on ALTER TABLE
To create a UNIQUE constraint on the “P_Id” column when the table is already created, use the following SQL:

MySQL / SQL Server / Oracle / MS Access:

ALTER TABLE Persons
ADD UNIQUE (P_Id)
To allow naming of a UNIQUE constraint, and for defining a UNIQUE constraint on multiple columns, use the following SQL syntax:

MySQL / SQL Server / Oracle / MS Access:

ALTER TABLE Persons
ADD CONSTRAINT uc_PersonID UNIQUE (P_Id,LastName)
To DROP a UNIQUE Constraint
To drop a UNIQUE constraint, use the following SQL:

MySQL:

ALTER TABLE Persons
DROP INDEX uc_PersonID
SQL Server / Oracle / MS Access:

ALTER TABLE Persons
DROP CONSTRAINT uc_PersonID

SQL PRIMARY KEY Constraint

The PRIMARY KEY constraint uniquely identifies each record in a database table.

Primary keys must contain UNIQUE values.

A primary key column cannot contain NULL values.

Most tables should have a primary key, and each table can have only ONE primary key.

SQL PRIMARY KEY Constraint on CREATE TABLE
The following SQL creates a PRIMARY KEY on the “P_Id” column when the “Persons” table is created:

MySQL:

CREATE TABLE Persons
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
PRIMARY KEY (P_Id)
)
SQL Server / Oracle / MS Access:

CREATE TABLE Persons
(
P_Id int NOT NULL PRIMARY KEY,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255)
)
To allow naming of a PRIMARY KEY constraint, and for defining a PRIMARY KEY constraint on multiple columns, use the following SQL syntax:

MySQL / SQL Server / Oracle / MS Access:

CREATE TABLE Persons
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
CONSTRAINT pk_PersonID PRIMARY KEY (P_Id,LastName)
)
Note: In the example above there is only ONE PRIMARY KEY (pk_PersonID). However, the VALUE of the primary key is made up of TWO COLUMNS (P_Id + LastName).
SQL PRIMARY KEY Constraint on ALTER TABLE
To create a PRIMARY KEY constraint on the “P_Id” column when the table is already created, use the following SQL:

MySQL / SQL Server / Oracle / MS Access:

ALTER TABLE Persons
ADD PRIMARY KEY (P_Id)
To allow naming of a PRIMARY KEY constraint, and for defining a PRIMARY KEY constraint on multiple columns, use the following SQL syntax:

MySQL / SQL Server / Oracle / MS Access:

ALTER TABLE Persons
ADD CONSTRAINT pk_PersonID PRIMARY KEY (P_Id,LastName)
Note: If you use the ALTER TABLE statement to add a primary key, the primary key column(s) must already have been declared to not contain NULL values (when the table was first created).

To DROP a PRIMARY KEY Constraint
To drop a PRIMARY KEY constraint, use the following SQL:

MySQL:

ALTER TABLE Persons
DROP PRIMARY KEY
SQL Server / Oracle / MS Access:

ALTER TABLE Persons
DROP CONSTRAINT pk_PersonID

SQL FOREIGN KEY Constraint

A FOREIGN KEY in one table points to a PRIMARY KEY in another table.

Let’s illustrate the foreign key with an example. Look at the following two tables:

The “Persons” table:

P_Id LastName FirstName Address City
1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes
3 Pettersen Kari Storgt 20 Stavanger
The “Orders” table:

O_Id OrderNo P_Id
1 77895 3
2 44678 3
3 22456 2
4 24562 1
Note that the “P_Id” column in the “Orders” table points to the “P_Id” column in the “Persons” table.

The “P_Id” column in the “Persons” table is the PRIMARY KEY in the “Persons” table.

The “P_Id” column in the “Orders” table is a FOREIGN KEY in the “Orders” table.

The FOREIGN KEY constraint is used to prevent actions that would destroy links between tables.

The FOREIGN KEY constraint also prevents invalid data from being inserted into the foreign key column, because it has to be one of the values contained in the table it points to.

SQL FOREIGN KEY Constraint on CREATE TABLE
The following SQL creates a FOREIGN KEY on the “P_Id” column when the “Orders” table is created:

MySQL:

CREATE TABLE Orders
(
O_Id int NOT NULL,
OrderNo int NOT NULL,
P_Id int,
PRIMARY KEY (O_Id),
FOREIGN KEY (P_Id) REFERENCES Persons(P_Id)
)
SQL Server / Oracle / MS Access:

CREATE TABLE Orders
(
O_Id int NOT NULL PRIMARY KEY,
OrderNo int NOT NULL,
P_Id int FOREIGN KEY REFERENCES Persons(P_Id)
)
To allow naming of a FOREIGN KEY constraint, and for defining a FOREIGN KEY constraint on multiple columns, use the following SQL syntax:

MySQL / SQL Server / Oracle / MS Access:

CREATE TABLE Orders
(
O_Id int NOT NULL,
OrderNo int NOT NULL,
P_Id int,
PRIMARY KEY (O_Id),
CONSTRAINT fk_PerOrders FOREIGN KEY (P_Id)
REFERENCES Persons(P_Id)
)
SQL FOREIGN KEY Constraint on ALTER TABLE
To create a FOREIGN KEY constraint on the “P_Id” column when the “Orders” table is already created, use the following SQL:

MySQL / SQL Server / Oracle / MS Access:

ALTER TABLE Orders
ADD FOREIGN KEY (P_Id)
REFERENCES Persons(P_Id)
To allow naming of a FOREIGN KEY constraint, and for defining a FOREIGN KEY constraint on multiple columns, use the following SQL syntax:

MySQL / SQL Server / Oracle / MS Access:

ALTER TABLE Orders
ADD CONSTRAINT fk_PerOrders
FOREIGN KEY (P_Id)
REFERENCES Persons(P_Id)
To DROP a FOREIGN KEY Constraint
To drop a FOREIGN KEY constraint, use the following SQL:

MySQL:

ALTER TABLE Orders
DROP FOREIGN KEY fk_PerOrders
SQL Server / Oracle / MS Access:

ALTER TABLE Orders
DROP CONSTRAINT fk_PerOrders

SQL CHECK Constraint

The CHECK constraint is used to limit the value range that can be placed in a column.

If you define a CHECK constraint on a single column it allows only certain values for this column.

If you define a CHECK constraint on a table it can limit the values in certain columns based on values in other columns in the row.

SQL CHECK Constraint on CREATE TABLE
The following SQL creates a CHECK constraint on the “P_Id” column when the “Persons” table is created. The CHECK constraint specifies that the column “P_Id” must only include integers greater than 0.

MySQL:

CREATE TABLE Persons
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
CHECK (P_Id>0)
)
SQL Server / Oracle / MS Access:

CREATE TABLE Persons
(
P_Id int NOT NULL CHECK (P_Id>0),
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255)
)
To allow naming of a CHECK constraint, and for defining a CHECK constraint on multiple columns, use the following SQL syntax:

MySQL / SQL Server / Oracle / MS Access:

CREATE TABLE Persons
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
CONSTRAINT chk_Person CHECK (P_Id>0 AND City=’Sandnes’)
)
SQL CHECK Constraint on ALTER TABLE
To create a CHECK constraint on the “P_Id” column when the table is already created, use the following SQL:

MySQL / SQL Server / Oracle / MS Access:

ALTER TABLE Persons
ADD CHECK (P_Id>0)
To allow naming of a CHECK constraint, and for defining a CHECK constraint on multiple columns, use the following SQL syntax:

MySQL / SQL Server / Oracle / MS Access:

ALTER TABLE Persons
ADD CONSTRAINT chk_Person CHECK (P_Id>0 AND City=’Sandnes’)
To DROP a CHECK Constraint
To drop a CHECK constraint, use the following SQL:

SQL Server / Oracle / MS Access:

ALTER TABLE Persons
DROP CONSTRAINT chk_Person
MySQL:

ALTER TABLE Persons
DROP CHECK chk_Person

SQL DEFAULT Constraint

The DEFAULT constraint is used to insert a default value into a column.

The default value will be added to all new records, if no other value is specified.

SQL DEFAULT Constraint on CREATE TABLE
The following SQL creates a DEFAULT constraint on the “City” column when the “Persons” table is created:

My SQL / SQL Server / Oracle / MS Access:

CREATE TABLE Persons
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255) DEFAULT ‘Sandnes’
)
The DEFAULT constraint can also be used to insert system values, by using functions like GETDATE():

CREATE TABLE Orders
(
O_Id int NOT NULL,
OrderNo int NOT NULL,
P_Id int,
OrderDate date DEFAULT GETDATE()
)
SQL DEFAULT Constraint on ALTER TABLE
To create a DEFAULT constraint on the “City” column when the table is already created, use the following SQL:

MySQL:

ALTER TABLE Persons
ALTER City SET DEFAULT ‘SANDNES’
SQL Server / MS Access:

ALTER TABLE Persons
ALTER COLUMN City SET DEFAULT ‘SANDNES’
Oracle:

ALTER TABLE Persons
MODIFY City DEFAULT ‘SANDNES’
To DROP a DEFAULT Constraint
To drop a DEFAULT constraint, use the following SQL:

MySQL:

ALTER TABLE Persons
ALTER City DROP DEFAULT
SQL Server / Oracle / MS Access:

ALTER TABLE Persons
ALTER COLUMN City DROP DEFAULT

The CREATE INDEX statement

The CREATE INDEX statement is used to create indexes in tables.

Indexes allow the database application to find data fast; without reading the whole table.

Indexes
An index can be created in a table to find data more quickly and efficiently.

The users cannot see the indexes, they are just used to speed up searches/queries.

Note: Updating a table with indexes takes more time than updating a table without (because the indexes also need an update). So you should only create indexes on columns (and tables) that will be frequently searched against.

SQL CREATE INDEX Syntax
Creates an index on a table. Duplicate values are allowed:

CREATE INDEX index_name
ON table_name (column_name)
SQL CREATE UNIQUE INDEX Syntax
Creates a unique index on a table. Duplicate values are not allowed:

CREATE UNIQUE INDEX index_name
ON table_name (column_name)
Note: The syntax for creating indexes varies amongst different databases. Therefore: Check the syntax for creating indexes in your database.

CREATE INDEX Example
The SQL statement below creates an index named “PIndex” on the “LastName” column in the “Persons” table:

CREATE INDEX PIndex
ON Persons (LastName)
If you want to create an index on a combination of columns, you can list the column names within the parentheses, separated by commas:

CREATE INDEX PIndex
ON Persons (LastName, FirstName)

SQL DROP INDEX, DROP TABLE, and DROP DATABASE

Indexes, tables, and databases can easily be deleted/removed with the DROP statement.

The DROP INDEX Statement
The DROP INDEX statement is used to delete an index in a table.

DROP INDEX Syntax for MS Access:
DROP INDEX index_name ON table_name
DROP INDEX Syntax for MS SQL Server:
DROP INDEX table_name.index_name
DROP INDEX Syntax for DB2/Oracle:
DROP INDEX index_name
DROP INDEX Syntax for MySQL:
ALTER TABLE table_name DROP INDEX index_name
The DROP TABLE Statement
The DROP TABLE statement is used to delete a table.

DROP TABLE table_name
The DROP DATABASE Statement
The DROP DATABASE statement is used to delete a database.

DROP DATABASE database_name
The TRUNCATE TABLE Statement
What if we only want to delete the data inside the table, and not the table itself?

Then, use the TRUNCATE TABLE statement:

TRUNCATE TABLE table_name

The ALTER TABLE Statement

The ALTER TABLE statement is used to add, delete, or modify columns in an existing table.

SQL ALTER TABLE Syntax
To add a column in a table, use the following syntax:

ALTER TABLE table_name
ADD column_name datatype
To delete a column in a table, use the following syntax (notice that some database systems don’t allow deleting a column):

ALTER TABLE table_name
DROP COLUMN column_name
To change the data type of a column in a table, use the following syntax:

SQL Server / MS Access:

ALTER TABLE table_name
ALTER COLUMN column_name datatype
My SQL / Oracle (prior version 10G):

ALTER TABLE table_name
MODIFY COLUMN column_name datatype
Oracle 10G and later:

ALTER TABLE table_name
MODIFY column_name datatype

SQL AUTO INCREMENT Field

Auto-increment allows a unique number to be generated when a new record is inserted into a table.

AUTO INCREMENT a Field
Very often we would like the value of the primary key field to be created automatically every time a new record is inserted.

We would like to create an auto-increment field in a table.

Syntax for MySQL
The following SQL statement defines the “ID” column to be an auto-increment primary key field in the “Persons” table:

CREATE TABLE Persons
(
ID int NOT NULL AUTO_INCREMENT,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
PRIMARY KEY (ID)
)
MySQL uses the AUTO_INCREMENT keyword to perform an auto-increment feature.

By default, the starting value for AUTO_INCREMENT is 1, and it will increment by 1 for each new record.

To let the AUTO_INCREMENT sequence start with another value, use the following SQL statement:

ALTER TABLE Persons AUTO_INCREMENT=100
To insert a new record into the “Persons” table, we will NOT have to specify a value for the “ID” column (a unique value will be added automatically):

INSERT INTO Persons (FirstName,LastName)
VALUES (‘Lars’,’Monsen’)
The SQL statement above would insert a new record into the “Persons” table. The “ID” column would be assigned a unique value. The “FirstName” column would be set to “Lars” and the “LastName” column would be set to “Monsen”.

Syntax for SQL Server
The following SQL statement defines the “ID” column to be an auto-increment primary key field in the “Persons” table:

CREATE TABLE Persons
(
ID int IDENTITY(1,1) PRIMARY KEY,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255)
)
The MS SQL Server uses the IDENTITY keyword to perform an auto-increment feature.

In the example above, the starting value for IDENTITY is 1, and it will increment by 1 for each new record.

Tip: To specify that the “ID” column should start at value 10 and increment by 5, change it to IDENTITY(10,5).

To insert a new record into the “Persons” table, we will NOT have to specify a value for the “ID” column (a unique value will be added automatically):

INSERT INTO Persons (FirstName,LastName)
VALUES (‘Lars’,’Monsen’)
The SQL statement above would insert a new record into the “Persons” table. The “ID” column would be assigned a unique value. The “FirstName” column would be set to “Lars” and the “LastName” column would be set to “Monsen”.

Syntax for Access
The following SQL statement defines the “ID” column to be an auto-increment primary key field in the “Persons” table:

CREATE TABLE Persons
(
ID Integer PRIMARY KEY AUTOINCREMENT,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255)
)
The MS Access uses the AUTOINCREMENT keyword to perform an auto-increment feature.

By default, the starting value for AUTOINCREMENT is 1, and it will increment by 1 for each new record.

Tip: To specify that the “ID” column should start at value 10 and increment by 5, change the autoincrement to AUTOINCREMENT(10,5).

To insert a new record into the “Persons” table, we will NOT have to specify a value for the “ID” column (a unique value will be added automatically):

INSERT INTO Persons (FirstName,LastName)
VALUES (‘Lars’,’Monsen’)
The SQL statement above would insert a new record into the “Persons” table. The “P_Id” column would be assigned a unique value. The “FirstName” column would be set to “Lars” and the “LastName” column would be set to “Monsen”.

Syntax for Oracle
In Oracle the code is a little bit more tricky.

You will have to create an auto-increment field with the sequence object (this object generates a number sequence).

Use the following CREATE SEQUENCE syntax:

CREATE SEQUENCE seq_person
MINVALUE 1
START WITH 1
INCREMENT BY 1
CACHE 10
The code above creates a sequence object called seq_person, that starts with 1 and will increment by 1. It will also cache up to 10 values for performance. The cache option specifies how many sequence values will be stored in memory for faster access.

To insert a new record into the “Persons” table, we will have to use the nextval function (this function retrieves the next value from seq_person sequence):

INSERT INTO Persons (ID,FirstName,LastName)
VALUES (seq_person.nextval,’Lars’,’Monsen’)
The SQL statement above would insert a new record into the “Persons” table. The “ID” column would be assigned the next number from the seq_person sequence. The “FirstName” column would be set to “Lars” and the “LastName” column would be set to “Monsen”.

SQL Views

A view is a virtual table.

This chapter shows how to create, update, and delete a view.

SQL CREATE VIEW Statement
In SQL, a view is a virtual table based on the result-set of an SQL statement.

A view contains rows and columns, just like a real table. The fields in a view are fields from one or more real tables in the database.

You can add SQL functions, WHERE, and JOIN statements to a view and present the data as if the data were coming from one single table.

SQL CREATE VIEW Syntax
CREATE VIEW view_name AS
SELECT column_name(s)
FROM table_name
WHERE condition
Note: A view always shows up-to-date data! The database engine recreates the data, using the view’s SQL statement, every time a user queries a view.

SQL CREATE VIEW Examples
If you have the Northwind database you can see that it has several views installed by default.

The view “Current Product List” lists all active products (products that are not discontinued) from the “Products” table. The view is created with the following SQL:

CREATE VIEW [Current Product List] AS
SELECT ProductID,ProductName
FROM Products
WHERE Discontinued=No
We can query the view above as follows:

SELECT * FROM [Current Product List]
Another view in the Northwind sample database selects every product in the “Products” table with a unit price higher than the average unit price:

CREATE VIEW [Products Above Average Price] AS
SELECT ProductName,UnitPrice
FROM Products
WHERE UnitPrice>(SELECT AVG(UnitPrice) FROM Products)
We can query the view above as follows:

SELECT * FROM [Products Above Average Price]
Another view in the Northwind database calculates the total sale for each category in 1997. Note that this view selects its data from another view called “Product Sales for 1997”:

CREATE VIEW [Category Sales For 1997] AS
SELECT DISTINCT CategoryName,Sum(ProductSales) AS CategorySales
FROM [Product Sales for 1997]
GROUP BY CategoryName
We can query the view above as follows:

SELECT * FROM [Category Sales For 1997]
We can also add a condition to the query. Now we want to see the total sale only for the category “Beverages”:

SELECT * FROM [Category Sales For 1997]
WHERE CategoryName=’Beverages’
SQL Updating a View
You can update a view by using the following syntax:

SQL CREATE OR REPLACE VIEW Syntax
CREATE OR REPLACE VIEW view_name AS
SELECT column_name(s)
FROM table_name
WHERE condition
Now we want to add the “Category” column to the “Current Product List” view. We will update the view with the following SQL:

CREATE OR REPLACE VIEW [Current Product List] AS
SELECT ProductID,ProductName,Category
FROM Products
WHERE Discontinued=No
SQL Dropping a View
You can delete a view with the DROP VIEW command.

SQL DROP VIEW Syntax
DROP VIEW view_name

SQL Dates

Note The most difficult part when working with dates is to be sure that the format of the date you are trying to insert, matches the format of the date column in the database.
As long as your data contains only the date portion, your queries will work as expected. However, if a time portion is involved, it gets complicated.

Before talking about the complications of querying for dates, we will look at the most important built-in functions for working with dates.

MySQL Date Functions
The following table lists the most important built-in date functions in MySQL:

Function Description
NOW() Returns the current date and time
CURDATE() Returns the current date
CURTIME() Returns the current time
DATE() Extracts the date part of a date or date/time expression
EXTRACT() Returns a single part of a date/time
DATE_ADD() Adds a specified time interval to a date
DATE_SUB() Subtracts a specified time interval from a date
DATEDIFF() Returns the number of days between two dates
DATE_FORMAT() Displays date/time data in different formats
SQL Server Date Functions
The following table lists the most important built-in date functions in SQL Server:

Function Description
GETDATE() Returns the current date and time
DATEPART() Returns a single part of a date/time
DATEADD() Adds or subtracts a specified time interval from a date
DATEDIFF() Returns the time between two dates
CONVERT() Displays date/time data in different formats
SQL Date Data Types
MySQL comes with the following data types for storing a date or a date/time value in the database:

DATE – format YYYY-MM-DD
DATETIME – format: YYYY-MM-DD HH:MI:SS
TIMESTAMP – format: YYYY-MM-DD HH:MI:SS
YEAR – format YYYY or YY
SQL Server comes with the following data types for storing a date or a date/time value in the database:

DATE – format YYYY-MM-DD
DATETIME – format: YYYY-MM-DD HH:MI:SS
SMALLDATETIME – format: YYYY-MM-DD HH:MI:SS
TIMESTAMP – format: a unique number
Note: The date types are chosen for a column when you create a new table in your database!

For an overview of all data types available, go to our complete Data Types reference.

SQL Working with Dates
Note You can compare two dates easily if there is no time component involved!
Assume we have the following “Orders” table:

OrderId ProductName OrderDate
1 Geitost 2008-11-11
2 Camembert Pierrot 2008-11-09
3 Mozzarella di Giovanni 2008-11-11
4 Mascarpone Fabioli 2008-10-29
Now we want to select the records with an OrderDate of “2008-11-11” from the table above.

We use the following SELECT statement:

SELECT * FROM Orders WHERE OrderDate=’2008-11-11′
The result-set will look like this:

OrderId ProductName OrderDate
1 Geitost 2008-11-11
3 Mozzarella di Giovanni 2008-11-11
Now, assume that the “Orders” table looks like this (notice the time component in the “OrderDate” column):

OrderId ProductName OrderDate
1 Geitost 2008-11-11 13:23:44
2 Camembert Pierrot 2008-11-09 15:45:21
3 Mozzarella di Giovanni 2008-11-11 11:12:01
4 Mascarpone Fabioli 2008-10-29 14:56:59
If we use the same SELECT statement as above:

SELECT * FROM Orders WHERE OrderDate=’2008-11-11′
we will get no result! This is because the query is looking only for dates with no time portion.

Tip: If you want to keep your queries simple and easy to maintain, do not allow time components in your dates!

SQL NULL Value

NULL values represent missing unknown data.

By default, a table column can hold NULL values.

This chapter will explain the IS NULL and IS NOT NULL operators.

SQL NULL Values
If a column in a table is optional, we can insert a new record or update an existing record without adding a value to this column. This means that the field will be saved with a NULL value.

NULL values are treated differently from other values.

NULL is used as a placeholder for unknown or inapplicable values.

Note Note: It is not possible to compare NULL and 0; they are not equivalent.
SQL Working with NULL Values
Look at the following “Persons” table:

P_Id LastName FirstName Address City
1 Hansen Ola Sandnes
2 Svendson Tove Borgvn 23 Sandnes
3 Pettersen Kari Stavanger
Suppose that the “Address” column in the “Persons” table is optional. This means that if we insert a record with no value for the “Address” column, the “Address” column will be saved with a NULL value.

How can we test for NULL values?

It is not possible to test for NULL values with comparison operators, such as =, <, or <>.

We will have to use the IS NULL and IS NOT NULL operators instead.

SQL IS NULL
How do we select only the records with NULL values in the “Address” column?

We will have to use the IS NULL operator:

SELECT LastName,FirstName,Address FROM Persons
WHERE Address IS NULL
The result-set will look like this:

LastName FirstName Address
Hansen Ola
Pettersen Kari
Note Tip: Always use IS NULL to look for NULL values.
SQL IS NOT NULL
How do we select only the records with no NULL values in the “Address” column?

We will have to use the IS NOT NULL operator:

SELECT LastName,FirstName,Address FROM Persons
WHERE Address IS NOT NULL
The result-set will look like this:

LastName FirstName Address
Svendson Tove Borgvn 23
In the next chapter we will look at the ISNULL(), NVL(), IFNULL() and COALESCE() functions.

SQL General Data Types

A data type defines what kind of value a column can contain.

SQL General Data Types
Each column in a database table is required to have a name and a data type.

SQL developers have to decide what types of data will be stored inside each and every table column when creating a SQL table. The data type is a label and a guideline for SQL to understand what type of data is expected inside of each column, and it also identifies how SQL will interact with the stored data.

The following table lists the general data types in SQL:

Data type Description
CHARACTER(n) Character string. Fixed-length n
VARCHAR(n) or
CHARACTER VARYING(n) Character string. Variable length. Maximum length n
BINARY(n) Binary string. Fixed-length n
BOOLEAN Stores TRUE or FALSE values
VARBINARY(n) or
BINARY VARYING(n) Binary string. Variable length. Maximum length n
INTEGER(p) Integer numerical (no decimal). Precision p
SMALLINT Integer numerical (no decimal). Precision 5
INTEGER Integer numerical (no decimal). Precision 10
BIGINT Integer numerical (no decimal). Precision 19
DECIMAL(p,s) Exact numerical, precision p, scale s. Example: decimal(5,2) is a number that has 3 digits before the decimal and 2 digits after the decimal
NUMERIC(p,s) Exact numerical, precision p, scale s. (Same as DECIMAL)
FLOAT(p) Approximate numerical, mantissa precision p. A floating number in base 10 exponential notation. The size argument for this type consists of a single number specifying the minimum precision
REAL Approximate numerical, mantissa precision 7
FLOAT Approximate numerical, mantissa precision 16
DOUBLE PRECISION Approximate numerical, mantissa precision 16
DATE Stores year, month, and day values
TIME Stores hour, minute, and second values
TIMESTAMP Stores year, month, day, hour, minute, and second values
INTERVAL Composed of a number of integer fields, representing a period of time, depending on the type of interval
ARRAY A set-length and ordered collection of elements
MULTISET A variable-length and unordered collection of elements
XML Stores XML data
SQL Data Type Quick Reference
However, different databases offer different choices for the data type definition.

The following table shows some of the common names of data types between the various database platforms:

Data type Access SQLServer Oracle MySQL PostgreSQL
boolean Yes/No Bit Byte N/A Boolean
integer Number (integer) Int Number Int
Integer Int
Integer
float Number (single) Float
Real Number Float Numeric
currency Currency Money N/A N/A Money
string (fixed) N/A Char Char Char Char
string (variable) Text (<256)
Memo (65k+) Varchar Varchar
Varchar2 Varchar Varchar
binary object OLE Object Memo Binary (fixed up to 8K)
Varbinary (<8K)
Image (<2GB) Long
Raw Blob
Text Binary
Varbinary
Note
Note: Data types might have different names in different database. And even if the name is the same, the size and other details may be different! Always check the documentation!

SQL COUNT()

The COUNT() function returns the number of rows that matches a specified criteria.

SQL COUNT(column_name) Syntax
The COUNT(column_name) function returns the number of values (NULL values will not be counted) of the specified column:

SELECT COUNT(column_name) FROM table_name;
SQL COUNT(*) Syntax
The COUNT(*) function returns the number of records in a table:

SELECT COUNT(*) FROM table_name;
SQL COUNT(DISTINCT column_name) Syntax
The COUNT(DISTINCT column_name) function returns the number of distinct values of the specified column:

SELECT COUNT(DISTINCT column_name) FROM table_name;
Note: COUNT(DISTINCT) works with ORACLE and Microsoft SQL Server, but not with Microsoft Access.

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