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Explain constraints and its types.

Constraints Overview:

  • Definition: Constraints are rules used for optimizing databases and ensuring data integrity during updates, deletes, or inserts into a table.
  • Purpose: Maintain data integrity by setting limits on data or types of data that can be manipulated in a table.

Types of Constraints:


  • Purpose: Ensures a column cannot have NULL values.
  • Example: If a column has a NOT NULL constraint, it means you cannot leave it empty when adding a new record.


  • Purpose: Requires a column or set of columns to have unique values.
  • Example: If a column has a UNIQUE constraint, no two records in the table can have the same value in that column.


  • Purpose: Provides a default value to a column if none is specified during an insert.
  • Example: If a DEFAULT constraint is set for a column with a default value of 0, if no value is provided during an insert, it will automatically be set to 0.


  • Purpose: Defines a range of acceptable values for a column.
  • Example: If a CHECK constraint is set for a column with a range of 1 to 100, any value outside this range will be rejected.

5. Key Constraints:

a. Primary Key:

  • Purpose: Uniquely identifies each record in a table.
  • Conditions: Must have unique values and cannot contain NULL.
  • Example: Social Security Number in an employee database.

b. Foreign Key:

  • Purpose: Establishes a link between tables by referencing the primary key of another table.
  • Example: If there’s a foreign key in an “Orders” table pointing to the “CustomerID” in a “Customers” table, it creates a relationship between them.

6. Domain Constraints:

  • Purpose: Enforces the data type for each column.
  • Example: If a column is set to store only integers, it won’t accept values like text or dates.

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