Constraints

There are some constraints in SQL that can change how data is stored in a SQL Table

• NOT NULL : Cannot store null values in that column
• PRIMARY KEY
• FOREIGN KEY
• CHECK
• DEFAULT

CREATE TABLE student (ID int NOT NULL, name VARCHAR(10) NOT NULL);

Primary Key

CREATE TABLE Persons (
    Personid int NOT NULL AUTO_INCREMENT,
    LastName varchar(255) NOT NULL,
    FirstName varchar(255),
    Age int,
    PRIMARY KEY (Personid)
);

Foreign Key

Connects data of one table to another table.

CREATE TABLE orders
(
	oid INT NOT NULL UNIQUE,
	cid INT,
	PRIMARY KEY (oid),
	FOREIGN KEY (cid) REFERENCES Customers(cid); //HERE Customers is the Table name
																							 //Which has cid as primary key
);

Check

CREATE TABLE student
(
	id INT NOT NULL,
	name VARCHAR(30),
	age int CHECK(AGE>=18)
);

Default

CREATE TABLE student
(
	id INT NOT NULL,
	name VARCHAR(30),
	age int DEFAULT 18
);