Integrity Constraints

Integrity constraint


It is the mechanism or condition used to prevent invalid data entry into the relation (table). It applies rules on the table.

Three types of integrity constraints are:
  1. Domain integrity constraints: Set a range on any relation that will prevent the user from performing any invalid manipulation.
    1. Not null constraint
    2. Check constraint
  2. Entity integrityh constraints: Applies on relation or table.
    1. Unique key constraint
    2. Primary key constraint
  3. Referential integrity constraints: Applies condition in two tables.
    1. Foreign key constraint
1. Not null: Not allow null values.
2. Check: Specify condition that each tuple must satisfy.
3. Unique key: Prevent duplicate values, allow NULL values.
4. Primary key: Prevent duplicate values, don’t allow null values.
5. Foreign key: Tuples in the referencing relation R1 have attribute (Foreign key attribute) that reference the primary key attribute of the referenced relation R2

Check constraint

The CHECK constraint is used to limit the value range that can be placed in a column.
The  CHECK constraint allows only certain values for a column.

How to create a CHECK constraint?

Check constraint can be created in this way:

CREATE TABLE Student(
RollNumber INT NOT NULL,
Name varchar(255), 
Age INT,
CHECK (Age>=18)
);

How to delete a CHECK constraint?

ALTER TABLE Student DROP CHECK Age;

Referenctial integrity

Referential Integrity ensures that a value that appears in one relation for a given set of attributes also appears for a certain set of attributes in another relation.
Example: 
If “CSE” is a branch name appearing in one of the tuples in the College relation (table), then there exists a tuple in the Department relation (table) for branch “CSE”.
Example in SQL:
create table customer
(customer-name           char(20),
customer-street            char(30),
customer-city  char(30),
primary key (customer-name))

create table depositor
(customer-name           char(20),
account-number          char(10),
primary key (customer-name, account-number),
foreign key (customer-name) references customer)
Check out this video lecture, for more info on integrity constraints


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  2. Louden, “Programming Languages: Principles & Practices” , Cengage Learning 
  3. Tucker, “Programming Languages: Principles and paradigms “, Tata McGraw –Hill. 
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