Primary key / Super Key / Candidate Key in SQL Server with Examples
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A Primary Key is a column or field in a table that makes rows or records unique in the table. Primary Keys contains all unique values with no duplicate or NULL values.
A Table can have only one Primary Key.
Primary key can be defined on single or multiple columns. Primary Key with a combination of multiple columns called Composite primary key.
The Primary key can have a maximum of 16 columns.
The system will not allow inserting a row with a primary key that already exists in the table.
You can create a primary key on the existing table However existing values of the selected column should have unique and Zero Nulls.
Primary Key creates clustered index by default.
You can force Primary Key to be created with non clustered index or when table already has clustered index, SQL Server itself create a non clustered index for the primary key.
While Creating Primary Key on existing column, Selected columns must are set to Not Null.
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