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In SQL, constraints are rules that are enforced on a table’s data to maintain the integrity and accuracy of the data. Constraints can be applied to one or more columns in a table, and they can be added or removed as needed.
Here are some of the most common types of constraints in SQL:
- Primary key constraint: This constraint ensures that each row in a table is uniquely identifiable by a specific column or set of columns and that the values in that column or columns cannot be null.
- Foreign key constraint: This constraint ensures that the values in a column or set of columns in one table correspond to values in a column or set of columns in another table.
- Unique constraint: This constraint ensures that the values in a column or set of columns are unique across all rows in the table and that the values cannot be null.
- Check constraint: This constraint ensures that the values in a column or set of columns meet a specific condition or set of conditions.
- Not null constraint: This constraint ensures that a column or set of columns cannot contain null values.
Constraints help to ensure the accuracy and consistency of data in a database.
By applying constraints to a table, you can ensure that the data in the table is always valid and that data is not accidentally deleted or altered.
Constraints can also help to improve database performance by allowing the database to optimize query execution based on the rules that are defined in the constraints.
