What is a constraint in SQL? Name a few.

A constraint in SQL defines rules or restrictions that apply to data in a table, ensuring data integrity. Common constraints include:

• PRIMARY KEY: Ensures the values’ uniqueness in a column.
• FOREIGN KEY: Enforces referential integrity between tables.
• UNIQUE: Ensures the uniqueness of values in a column.
• CHECK: Defines a condition that data must meet to be inserted or updated.
• NOT NULL: Ensures that there are no NULL values in a column.