SQL Table Constraints

24th December 2022 · sql, database

NOT NULL constraint enforces a column to NOT accept NULL values.

CREATE TABLE User (
 ID int NOT NULL,
 LastName varchar(255) NOT NULL,
 FirstName varchar(255) NOT NULL,
 Age int
);

UNIQUE constraint ensures that all values in a column are different.

CREATE TABLE User (
 ID int NOT NULL UNIQUE,
 LastName varchar(255) NOT NULL,
 FirstName varchar(255)
);

PRIMARY KEY constraint uniquely identifies each record in a table.

CREATE TABLE User (
 ID int NOT NULL PRIMARY KEY,
 LastName varchar(255) NOT NULL,
 FirstName varchar(255)
);

FOREIGN KEY constraint is used to prevent actions that would destroy links between tables.

CREATE TABLE Orders (
 OrderID int NOT NULL PRIMARY KEY,
 OrderNumber int NOT NULL,
 UserID int FOREIGN KEY REFERENCES User(UserID)
);

CHECK constraint is used to limit the value range that can be placed in a column.

CREATE TABLE Persons (
 ID int NOT NULL,
 LastName varchar(255) NOT NULL,
 FirstName varchar(255),
 Age int CHECK (Age>=18)
);