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Constraints in sql server

Constraints - Constraints are the rules enforced on data columns on table. This ensures the accuracy and reliability of the data in the database. Constraints define some conditions that restricts the column to remain true while inserting or updating or deleting data in column.

Constraints can be defined in two ways 
1) The constraints can be specified immediately after the column definition.
This is called column-level definition. 
2) The constraints can be specified after all the columns are defined.
This is called table-level definition. 

Types of Constraints in SQL-Server

NOT NULL- This constraint is responsible for a column to confirm that a column cannot have a null value.

DEFAULTThis constraint provides a default value when specified non for this column.

UNIQUE- This constraint insure that each row for the column has different value.

PRIMARY KEY- Primary key constraint is a combination of a NOT NULL constraint and a UNIQUE constraint. This constraint ensures that the specific column for a table have an unique identity.

FOREIGN KEY- this constraints responsible to uniquely identified a rows/records in any another database table.

CHECK- The CHECK responsible to enables a condition to check the value being entered into a record

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