Constraints in SQL

Constraints in SQL are rules or conditions that are enforced on data within a database table to maintain the integrity, consistency, and validity of the data. Constraints define certain limitations or requirements that data must adhere to when it is inserted, updated, or deleted in a table. These constraints help ensure data quality and accuracy. Here are some common types of constraints in SQL:- 

1. NOT NULL Constraint:-  This constraint ensures that a column cannot contain NULL values, meaning that every row must have a value for that column.

2. UNIQUE Constraint:-  This constraint ensures that all values in a column or a group of columns are unique across all the rows in the table. It prevents duplicate values in the specified column(s).

3. PRIMARY KEY Constraint:-  This constraint uniquely identifies each record in a table. It is a combination of the NOT NULL and UNIQUE constraints. A table can have only one primary key, and it must contain unique values for each row.

4. FOREIGN KEY Constraint:-  This constraint establishes a relationship between two tables by enforcing referential integrity. It ensures that values in a column (or group of columns) of one table match values in another table's primary key or unique key.

5. CHECK Constraint:-  This constraint specifies a condition that must be met for the data in a column. It allows you to define custom rules to restrict the range of allowable values in a column.

6. DEFAULT Constraint:-  This constraint provides a default value for a column if no value is explicitly specified during an INSERT operation. It ensures that the column always has a value, even if it is not provided explicitly.

7. CHECK Constraint:-  This constraint specifies a condition that must be met for the data in a column. It allows you to define custom rules to restrict the range of allowable values in a column.

Constraints play a crucial role in maintaining data integrity and consistency within a database. They help prevent data entry errors, enforce business rules, and ensure that the database remains accurate and reliable over time.