Constraint is used to define rules on what values are inserted in the table. Constraint plays an important role in ensuring the accuracy and integrity of the data inserted into a table.

Commonly used constraints are as follows

1. NOT NULL

If a column is specified as NOT NULL while defining the table, then the column must not have NULL values while inserting data into the table.

2. UNIQUE

If a column is specified as UNIQUE while defining the table, then the column must have different values for all the rows present in the table.

3. PRIMARY KEY

Primary key is a column or set of columns which is used to uniquely identify a row in the database table. In the below customer table, You can either choose InsuranceID or National Insurance Number(NIN) as primary keys, but Insurance ID is preferable as NIN can be considered as personal information.

Table 1

customer(InsuranceID, Name, DOB, NIN, Location)

Table 2

patient(HospitalID, Name, DOB, InsuranceID)

4. FOREIGN KEY

Foreign key is a column in one table which references a primary key in other table. In the above two tables, InsuranceID of patient table is a foreign key which can be referenced to InsuranceID in customer Table.

5. CHECK

checks for a specific condition while inserting data into the table.

6. DEFAULT

If a value is not specified for a column whose default value is mentioned while creating the table, then the default value is inserted while inserting data.