MSSQL Server Constraints

There are six constraints which are NOT NULL, UNIQUE, PRIMARY KEY, FOREIGN KEY, CHECK, DEFAULT. I will explain all of them with giving example for each one.

NOT NULL : If you use this constraint for a column then this column cannot contain null value.
UNIQUE : Ensures that every column has a unique value.
PRIMARY KEY : Primary key has two specific rules If a column has primary key constraint then it must be not null and unique. When we use primary key it will help us to find record which we are searching correctly.
FOREIGN KEY : This constraint ensures the value in a column in a table  to match value in a column in another table.
CHECK :Ensures which the value in a column meets a specific condition.
DEFAULT : It is using for put a default value when create  new record for a column.

I created three entity  in visual paradigm to give examples

Using Constraints while creating table

NOT NULL

CREATE TABLE STUDENTS(
StudentName varchar(25) NOT NULL,
StudentLastName varchar(25) NOT NULL
);

UNIQUE

CREATE TABLE STUDENTS(
StudentID int NOT NULL UNIQUE,
StudentName varchar(25) NOT NULL,
StudentLastName varchar(25) NOT NULL
);

PRIMARY KEY

CREATE TABLE STUDENTS(
StudentID int IDENTITY(1,1) NOT NULL,
StudentName varchar(25) NOT NULL,
StudentLastName varchar(25) NOT NULL
);

FOREIGN KEY

CREATE TABLE SCHEDULE(
ScheduleID int IDENTITY(1,1) NOT NULL,
StudentID int FOREIGN KEY REFERENCES STUDENTS(StudentID),
LectureID int FOREIGN KEY REFERENCES LECTURES(LectureID),

);

CHECK

CREATE TABLE STUDENTS(
StudentID int IDENTITY(1,1) NOT NULL,
StudentName varchar(25) NOT NULL,
StudentLastName varchar(25) NOT NULL,
StudentAge int CHECK (StudentAge>0)
);

DEFAULT

CREATE TABLE STUDENTS(
StudentID int IDENTITY(1,1) NOT NULL,
StudentName varchar(25) NOT NULL,
StudentLastName varchar(25) NOT NULL,
StudentregistrationDate DEFAULT GETDATE()
);

Using constraints for already created table
UNIQUE

ALTER TABLE STUDENTS ADD UNIQUE (StudentID)

PRIMARY KEY

ALTER TABLE STUDENTS ADD PRIMARY KEY (StudentID)

FOREIGN KEY

ALTER TABLE SCHEDULE ADD FOREIGN KEY (StudentID) REFERENCES STUDENTS(StudentID)

CHECK

ALTER TABLE STUDENTS CHECK (StudentAge>0) 

DEFAUL

ALTER TABLE STUDENTS ALTER COLUMN StudentAge SET DEFAULT 0