SQL Table Constraints

Setting constraints in SQL tables

Author: Chris Lipinski, Published: 20221224

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) );

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