MySQL Constraints

Create Constraints

Constraints can be defined during table creation using the CREATE TABLE statement or added later using the ALTER TABLE statement.

Syntax

CREATE TABLE table_name (
    column1 datatype constraint,
    column2 datatype constraint,
    column3 datatype constraint,
    ….
);

MySQL Constraints

SQL constraints define rules for data within a table, ensuring data accuracy and reliability by limiting the types of data that can be entered. If data actions violate a constraint, the action is halted to maintain data integrity.

Constraints in SQL can be categorized as column-level or table-level. Column-level constraints are specific to individual columns, while table-level constraints apply to the entire table.

Commonly used constraints in SQL include:

NOT NULL: Ensures a column cannot have a NULL value.
UNIQUE: Ensures all values in a column are distinct.
PRIMARY KEY: Combines NOT NULL and UNIQUE, uniquely identifying each row in a table.
FOREIGN KEY: Prevents actions that would break relationships between tables.
CHECK: Ensures values in a column meet a specific condition.
DEFAULT: Sets a default value for a column if none is specified.
CREATE INDEX: Used to enhance data retrieval speed from the database.