A table usually has a column or combination of columns whose values uniquely identify each row in the table. This column (or columns) is called the primary key. Primary key ensures that no duplicate or null values are entered in the column (or columns) defined as primary key columns. This enforces integrity of the table. A primary key can be created by defining a primary key constraint while creating or altering a table. A table can have only one primary key constraint. As primary key constraints ensure uniqueness, they are often defined by using an identity column.
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