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A composite primary key is a primary key that consists of two or more columns. It is used to uniquely identify a row in a table, and it can be created by specifying multiple columns as the primary key when creating a table or by altering an existing table.
To master SQL Server composite primary keys, follow these steps:
- Understand the concept of primary keys: A primary key is a unique identifier for a row in a table. It must be unique and not null. When creating a table, you can specify one or more columns as the primary key.
- Determine the columns for the composite primary key: When creating a composite primary key, you need to determine which columns will be included in the key. The columns you choose should be unique and not null.
- Create the table with the composite primary key: When creating a table with a composite primary key, you can specify the columns that will make up the key in the CREATE TABLE statement. For example:CREATE TABLE MyTable ( Column1 INT, Column2 VARCHAR(50), Column3 DATE, CONSTRAINT PK_MyTable PRIMARY KEY (Column1, Column2, Column3) );
- Alter an existing table to add a composite primary key: If you need to add a composite primary key to an existing table, you can use the ALTER TABLE statement. For example:ALTER TABLE MyTable ADD CONSTRAINT PK_MyTable PRIMARY KEY (Column1, Column2, Column3);
- Use the composite primary key in queries: Once you have created the composite primary key, you can use it in queries to uniquely identify rows in the table. For example:SELECT * FROM MyTable WHERE Column1 = 1 AND Column2 = ‘Value’ AND Column3 = ‘2023-04-09’;
By mastering SQL Server composite primary keys, you can ensure the uniqueness and integrity of your data and improve the performance of your queries.