PRIMARY KEY Constraints
The primary key of a table is a column set that uniquely identifies each row of that table. See Database Design. You cannot define more than 1 primary key for a table. Primary keys, which are a logical construct in the relational model, are usually implemented physically as the unique primary index.
To create a composite, or multicolumn, primary key for a table, you must specify the PRIMARY KEY constraint at the table level, not the column level.
Columns defined with a data type from the following list cannot be a component of a PRIMARY KEY constraint.
When a table has a non‑unique primary index, you should consider defining its primary key explicitly using the PRIMARY KEY clause. Primary and other alternate keys are also used with foreign keys to enforce referential integrity relationships between tables (see “Standard Referential Integrity Constraints” on page 562, “Batch Referential Integrity Constraints” on page 562, and “Referential Constraints” on page 563).