- Introduction To SQL
- SQL Syntax
- SQL SELECT Statement
- SQL SELECT DISTINCT Statement
- SQL WHERE Clause
- SQL AND, OR And NOT Operators
- SQL ORDER BY Keyword
- SQL INSERT INTO Statement
- SQL NULL Values
- SQL UPDATE Statement
- SQL DELETE Statement
- SQL TOP, LIMIT, FETCH FIRST Or ROWNUM Clause
- SQL MIN() And MAX() Functions
- SQL COUNT(), AVG() And SUM() Functions
- SQL LIKE Operator
- SQL Wildcards
- SQL IN Operator
- SQL BETWEEN Operator
- SQL Aliases
- SQL Joins
- SQL INNER JOIN Keyword
- SQL LEFT JOIN Keyword
- SQL RIGHT JOIN Keyword
- SQL FULL OUTER JOIN Keyword
- SQL Self Join
- SQL UNION Operator
- SQL GROUP BY Statement
- SQL HAVING Clause
- SQL EXISTS Operator
- SQL ANY And ALL Operators
- SQL SELECT INTO Statement
- SQL INSERT INTO SELECT Statement
- SQL CASE Statement
- SQL NULL Functions
- SQL Stored Procedures For SQL Server
- SQL Comments
- SQL Operators
- SQL CREATE DATABASE Statement
- SQL DROP DATABASE Statement
- SQL BACKUP DATABASE For SQL Server
- SQL CREATE TABLE Statement
- SQL DROP TABLE Statement
- SQL ALTER TABLE Statement
- SQL Constraints
- SQL NOT NULL Constraint
- SQL UNIQUE Constraint
- SQL PRIMARY KEY Constraint
- SQL FOREIGN KEY Constraint
- ALTER TABLE Orders DROP CONSTRAINT FK_PersonOrder;
- SQL DEFAULT Constraint
- SQL CREATE INDEX Statement
- SQL AUTO INCREMENT Field
- SQL Working With Dates
- SQL Views
- SQL Injection
- SQL Hosting
- SQL Data Types For MySQL, SQL Server, And MS Access
SQL PRIMARY KEY Constraint
SQL PRIMARY KEY Constraint
The PRIMARY KEY constraint uniquely identifies each record in a table.
Primary keys must contain UNIQUE values, and cannot contain NULL values.
A table can have only ONE primary key; and in the table, this primary key can consist of single or multiple columns (fields).
Practice Excercise Practice now
SQL PRIMARY KEY On CREATE TABLE
The following SQL creates a PRIMARY KEY on the "ID" column when the "Persons" table is created:
MySQL:
ID int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Age int,
PRIMARY KEY (ID)
);
SQL Server / Oracle / MS Access:
ID int NOT NULL PRIMARY KEY,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Age int
);
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:
ID int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Age int,
CONSTRAINT PK_Person PRIMARY KEY (ID,LastName)
);
Note: In the example above there is only ONE
PRIMARY KEY (PK_Person). However, the VALUE of the primary key is made up of TWO COLUMNS (ID + LastName). Practice Excercise Practice now
SQL PRIMARY KEY On ALTER TABLE
To create a PRIMARY KEY constraint on the "ID" column when the table is already created, use the following SQL:
MySQL / SQL Server / Oracle / MS Access:
ADD PRIMARY KEY (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:
ADD CONSTRAINT PK_Person PRIMARY KEY (ID,LastName);
Note: If you use ALTER TABLE to add a primary key, the primary key column(s) must have been declared to not contain NULL values (when the table was first created).
Practice Excercise Practice now
DROP A PRIMARY KEY Constraint
To drop a PRIMARY KEY constraint, use the following SQL:
MySQL:
DROP PRIMARY KEY;
SQL Server / Oracle / MS Access:
DROP CONSTRAINT PK_Person;
Practice Excercise Practice now
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