This document discusses database constraints. It explains that constraints are used to specify rules for data in tables to maintain integrity. There are two levels of constraints - column and table level. The main types of constraints are NOT NULL, UNIQUE, CHECK, DEFAULT, and key constraints like PRIMARY KEY and FOREIGN KEY. Constraints enforce limits on the data, make columns non-nullable, enforce unique values, check value ranges, provide default values, and relate tables through primary and foreign keys. Maintaining constraints helps ensure only valid data is stored in the database.

1. Database Constraints
Constraints are used to specify rules for the
data in a table.
 It enforces limits to the data or type of data
that can be inserted/updated/deleted from a
table.

2. Purpose
• The main purpose of constraints is to maintain
the data integrity during database operations
(update/delete/insert) into a table.
• The term data integrity simply means that the
data stored in the table is valid.

3. Levels of Constraints
• There are two levels where a constraint can be
defined:
1. Column Level- A column level constraint
references a single column and is defined along
with the definition of the column.
2. Table Level- A table level constraint references
one or more columns and is defined separately
from the definitions of the columns. Normally, it
is written after all columns are defined.

4. Types of constraints
• NOT NULL
• UNIQUE
• CHECK
• DEFAULT
• Key Constraints – PRIMARY KEY, FOREIGN KEY

5. NOT NULL Constraint
NOT NULL constraint makes sure that a column does not hold NULL
value. Normally, when we don’t provide value for a particular column
while inserting a record into a table, it takes NULL value by default.
But, by specifying NOT NULL constraint, we can be sure that a
particular column(s) cannot have NULL values.
Ex:
CREATE TABLE STUDENT
(
ROLL_NO INT NOT NULL,
STU_NAME VARCHAR (35) NOT NULL,
STU_AGE INT NOT NULL,
STU_ADDRESS VARCHAR (235),
PRIMARY KEY (ROLL_NO)
);

6. UNIQUE Constraint
UNIQUE Constraint enforces a column or set of
columns to have unique values. If a column has
a UNIQUE constraint, it means that particular
column cannot have duplicate values in a table.
Ex:
CREATE TABLE STUDENT
(
ROLL_NO INT NOT NULL,
STU_NAME VARCHAR (35) NOT NULL UNIQUE,
STU_AGE INT NOT NULL,
STU_ADDRESS VARCHAR (35) UNIQUE,
PRIMARY KEY (ROLL_NO)
);

7. CHECK Constraint
CHECK constraint is used to restrict the value of a column
between a range. It performs check on the values, before storing
them into the database. Its like condition checking before saving
data into a column.
Ex:
CREATE TABLE STUDENT
(
ROLL_NO INT NOT NULL CHECK(ROLL_NO >1000) ,
STU_NAME VARCHAR (35) NOT NULL,
STU_AGE INT NOT NULL,
EXAM_FEE INT DEFAULT 10000,
STU_ADDRESS VARCHAR (35)
PRIMARY KEY (ROLL_NO
);

8. DEFAULT Constraint
The DEFAULT constraint provides a default value
to a column when there is no value provided
while inserting a record into a table.
Ex:
CREATE TABLE STUDENT
(
ROLL_NO INT NOT NULL,
STU_NAME VARCHAR (35) NOT NULL,
STU_AGE INT NOT NULL,
EXAM_FEE INT DEFAULT 10000,
STU_ADDRESS VARCHAR (35) ,
PRIMARY KEY (ROLL_NO)
);

9. Key Constraints
1. PRIMARY KEY:
• Uniquely identifies each record in a table. It must
have unique values and cannot contain nulls.
• In the below example the ROLL_NO field is marked as
primary key, that means the ROLL_NO field cannot
have duplicate and null values.
Ex:
CREATE TABLE STUDENT
(
ROLL_NO INT NOT NULL,
STU_NAME VARCHAR (35) NOT NULL UNIQUE,
STU_AGE INT NOT NULL,
STU_ADDRESS VARCHAR (35) UNIQUE,
PRIMARY KEY (ROLL_NO)
);

10. Key Constraints
2. FOREIGN KEY
• Foreign keys are the columns of a table that points to the primary
key of another table.
• It is used to relate two tables.
• It is also used to restrict actions that would destroy link between
table.
Ex:
CREATE table Order_Detail
(
order_id int PRIMARY KEY,
order_name varchar(60) NOT NULL,
c_id int FOREIGN KEY REFERENCES Customer_Detail(c_id)
);
In Customer_Detail table, c_id is the primary key which is set as foreign key
in Order_Detail table. The value that 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. This prevents invalid data to be inserted into c_id column
of Order_Detailtable.

11. THANK YOU