SQL Constraints

In SQL, a constraint is a rule that is defined to ensure the integrity of the data in a database. Constraints are used to enforce data validation and maintain data accuracy and consistency.

There are different types of constraints in SQL, which include:

1. NOT NULL Constraint: This constraint ensures that a column cannot contain NULL values. For example, if we define a column as NOT NULL, we are telling the database that it is mandatory to have a value in that column for every row.
2. UNIQUE Constraint: This constraint ensures that the values in a column are unique. For example, if we define a column as UNIQUE, we are telling the database that each value in that column must be unique and cannot be duplicated.
3. PRIMARY KEY Constraint: This constraint is used to uniquely identify each row in a table. For example, we can define a column as a primary key to ensure that each row in the table has a unique identifier.
4. FOREIGN KEY Constraint: This constraint is used to enforce a relationship between two tables. It ensures that the value in a column of one table exists in a specific column of another table. For example, if we have a “students” table and a “courses” table, we can use a foreign key constraint to ensure that each student in the “students” table is enrolled in a valid course in the “courses” table.
5. CHECK Constraint: This constraint is used to ensure that the values in a column meet a specific condition. For example, we can define a CHECK constraint to ensure that the values in a column fall within a certain range.
6. DEFAULT Constraint: This constraint is used to provide a default value for a column if no value is specified. For example, if we define a column with a default value of 0, then if no value is provided for that column, the database will automatically assign 0 as the default value.

Constraints are essential for maintaining data integrity and ensuring the accuracy and consistency of data in a database. When creating tables in SQL, it is recommended to define appropriate constraints to ensure data accuracy and consistency.