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- Create A Table
- Generate Primary Key When Creating A Table Sql Tutorial
- Generate Primary Key When Creating A Table Sql Pdf
- Sql Server Create Primary Key
Summary: in this tutorial, you will learn how to use the SQL Server PRIMARY KEY constraint to create a primary key for a table.
Introduction to SQL Server PRIMARY KEY constraint
A primary key is a column or a group of columns that uniquely identifies each row in a table. You create a primary key for a table by using the PRIMARY KEY constraint.
If the primary key consists of only one column, you can define use PRIMARY KEY constraint as a column constraint:
In case the primary key has two or more columns, you must use the PRIMARY KEY constraint as a table constraint:
Each table can contain only one primary key. All columns that participate in the primary key must be defined as NOT NULL. SQL Server automatically sets the NOT NULL constraint for all the primary key columns if the NOT NULL constraint is not specified for these columns.
SQL Server also automatically creates a unique clustered index (or a non-clustered index if specified as such) when you create a primary key.
SQL Server PRIMARY KEY constraint examples
The following example creates a table with a primary key that consists of one column:
In this sales.activities table, the activity_id column is the primary key column. It means the activity_id column contains unique values.
The IDENTITY property is used for the activity_id column to automatically generate unique integer values.
The following statement creates a new table named sales.participants whose primary key consists of two columns:
In this example, the values in either activity_id or customer_id column can be duplicate, but each combination of values from both columns must be unique.
Typically, a table always has a primary key defined at the time of creation. However, sometimes, an existing table may not have a primary key defined. In this case, you can add a primary key to the table by using the ALTER TABLE statement. Consider the following example:
The following statement creates a table without a primary key:
To make the event_id column as the primary key, you use the following ALTER TABLE statement:
Note that if the sales.events table already has data, before promoting the event_id column as the primary key, you must ensure that the values in the event_id are unique.
In this tutorial, you have learned how to use the SQL Server PRIMARY KEY constraint to create a primary key for a table.
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).
Create A Table
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:
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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).
Generate Primary Key When Creating A Table Sql Tutorial
SQL PRIMARY KEY on ALTER TABLE
Generate Primary Key When Creating A Table Sql Pdf
To create a PRIMARY KEY constraint on the 'ID' column when the table is already created, use the following SQL:
Sql Server Create Primary Key
MySQL / SQL Server / Oracle / MS Access:
To allow naming of a PRIMARY KEY constraint, and for defining a PRIMARY KEY constraint on multiple columns, use the following SQL syntax:
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MySQL / SQL Server / Oracle / MS Access:
ADD CONSTRAINT PK_Person PRIMARY KEY (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).
DROP a PRIMARY KEY Constraint
To drop a PRIMARY KEY constraint, use the following SQL:
MySQL:
SQL Server / Oracle / MS Access: