SQL Table Constraints

By Chris Lipinski · 24th December 2022 · 1 min read

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)
);