In the context of a Database Management System (DBMS), constraints are rules and restrictions that are enforced on the data in a database. These constraints ensure data integrity, accuracy, and reliability by governing what data can be entered, updated, or deleted in a database table. Here are the main types of constraints commonly used in databases:

Types:-

1. Primary Key Constraint

• Definition: Ensures that each row in a table has a unique and non-null value in the specified column(s). The primary key uniquely identifies each record in the table.
• Example: In a Students table, the student_id column could be the primary key, ensuring that no two students have the same ID.

2. Foreign Key Constraint

• Definition: Ensures that the value in a column (or a group of columns) matches a value in the primary key of another table, establishing a relationship between the two tables.
• Example: In an Enrollments table, the student_id column could be a foreign key that references the student_id in the Students table, ensuring that all student IDs in the Enrollments table are valid.

3. Unique Constraint

• Definition: Ensures that all values in a column (or a group of columns) are unique across the table, except for null values.
• Example: In a Users table, the email column could have a unique constraint to ensure that no two users have the same email address.

4. Not Null Constraint

• Definition: Ensures that a column cannot have null values, meaning every row must have a value in that column.