How to Create and Use Composite Primary Keys in SQL Server 2008?
Composite Primary Keys in SQL Server 2008
When designing tables in SQL Server, it is essential to define a primary key for efficient data retrieval and integrity maintenance. A primary key uniquely identifies each row in a table. In certain scenarios, it may be necessary to establish a composite primary key, which consists of multiple columns combined.
Creating a Composite Primary Key
The following steps guide you through creating a composite primary key in SQL Server 2008:
- Define the table structure: First, define the table's columns and data types.
create table my_table (
column_a integer not null,
column_b integer not null,
column_c varchar(50)
);- Specify the composite primary key: Use the PRIMARY KEY constraint to define the composite key. The primary key may comprise multiple columns separated by commas.
ALTER TABLE my_table ADD CONSTRAINT PK_my_table PRIMARY KEY (column_a, column_b);
This creates a primary key consisting of the column_a and column_b columns.
Example:
For instance, in a hypothetical "Orders" table containing columns like OrderID, CustomerID, and ProductID, you might set the OrderID as the primary key. However, to ensure uniqueness further, you could create a composite primary key using OrderID and CustomerID.
CREATE TABLE Orders (
OrderID int NOT NULL,
CustomerID int NOT NULL,
ProductID int NOT NULL,
OrderDate datetime,
PRIMARY KEY (OrderID, CustomerID)
);By implementing a composite primary key, you guarantee that each combination of OrderID and CustomerID is unique within the "Orders" table.
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