SQL Constraints

SQL constraints are used to specify rules for the data in a table.

Constraints can be specified when the table is created with the CREATE TABLE statement, or after the table is created with the ALTER TABLE statement.

Common Constraints

NOT NULL - Ensures that a column cannot have a NULL value
UNIQUE - Ensures that all values in a column are different
PRIMARY KEY - A combination of a NOT NULL and UNIQUE. Uniquely identifies each row in a table
FOREIGN KEY - Uniquely identifies a row/record in another table
CHECK - Ensures that all values in a column satisfy a specific condition
DEFAULT - Sets a default value for a column when no value is specified
CREATE INDEX - Used to create and retrieve data from the database very quickly