This document defines and describes different types of database constraints. It discusses table-level and column-level constraints. The most common constraints are listed as not null, primary key, unique key, and foreign key. Examples are provided for each constraint type to illustrate how they can be applied when creating or altering tables to maintain data integrity. A foreign key specifically relates data between two tables by referencing the primary key of another table.

1. CONSTRAINTS

2. DEFINITION:
 Constraints are rules to limit data that can go into a table ,to maintain integrity and
accuracy of the data into table .
 Constraints are divided into following two types :
 1-table level constraints
 2-column level constraints
 Constraints are used to make sure integrity of data is maintained in database.

3. FOLLOWING ARE THE MOST USED CONSTRAINTS :
 Not null
 Primary key
 Unique key
 Foreign key

4. NOT NULL:
 By applying not null constraint you are declaring that a not null value cannot be added.
 Create table student (s-id int Not Null ,Name varchar (60),Age int);
The above query shows that s-id student table will not take any Null
value.

5. UNIQUE CONSTRAINT:
 A unique constraint that a field or column will only have unique values , no value will
repeat.
 There will not be any duplicate value , this constraint can be applied on column level or
table level.
 Create table student (s-id int NOT NULL UNIQUE, NAME varchar(60),Age int );
 The above query insures that a s-id of student table will only take unique value ,there
also wont be a NULL value.
 Using UNIQUE constraint after table is created (column level):
 ALTER TABLE student ADD UNIQUE (s-id);

6. PRIMARY KEY :
 Primary key constraint uniquely identifies each round in a database.
 A primary key must contain unique value and it must not contain null value.
 Usually primary key is used to index the data inside the table.
 Using primary key constraint at table level
 Create table student (s-id int primary key, Name varchar (60)NOT NULL , Age int)
 The above command will create a PRIMARY KEY on the s-id.

7.  Using PRIMARY KEY constraint at Column Level
 Alter table student add PRIMARY KEY (s-id);
 The above command will create a PRIMARY KEY on the s-id.

8. FOREIGN KEY:
 FOREIGN KEY is used to relate two tables.
 FOREIGN KEY constraint is also used to restrict actions that would destroy links
between two tables.
 To understand FOREIGN KEY, lets see its use, with help of below tables:

9. CUSTOMER-DETAIL TABLE :
C-id Customer name Address
101 Adam England
102 Alex France
103 Stuart Germany

10. ORDER-DETAIL TABLE :
Order-id Order-Name C-id
10 order1 101
11 order2 103
12 order3 102

11.  In customer- detail table c-id is a PRIMARY KEY which is set as FOREIGN
KEY in order- detail table .
 The value which is entered in c-id which is set as FOREIGN KEY in order-
detail- table must be present in customer -detail table ,where it is set as
PRIMARY KEY.

12. USING FOREIGN KEY AT TABLE LEVEL :
create table Order-Detail(
order-id int PRIMARY KEY,
order-name varchar(60) NOT NULL,
c-id FOREIGN KEY REFERENCES Customer-Detail(c-id)
);
In this query ,c-id in the table order-detail is made as FOREIGN KEY ,which is a reference
of c-id column in customer-detail table.

13. USING FOREIGN KEY CONSTRAINT AT
COLUMN LEVEL:
 ALTER table order-Detail ADD FOREIGN KEY (c-id) REFERENCES customer-Detail(c-
id);