SQL Server interview questions | data integrity constraints

SQL Server interview questions | data integrity constraints

What is primary key?

A Primary Key in a table identifies each and every row uniquely.

It should not allow null values.

We could assign primary key on only column or more than one column also.

What is the difference between primary key and unique key?

Primary should not allow null; where as unique key will allow nulls.

By default Primary key is created as clustered index; whereas unique key is created as non clustered index.

What are the different levels of data integrity in SQL Server?

Entity Integrity, Domain Integrity, Referential integrity

Entity Integrity ensures that there are no duplicate rows in a table.

Domain Integrity enforces valid entries for a given column by restricting the type, the format, or the range of possible values.

Referential integrity ensures that rows cannot be deleted, which are used by other records.

Explain different constraints to maintain data integrity in SQL Server?

Check constraints: 
Check constraints will be useful to limit the range of possible values in a column.
We could create check constraints at two different levels

a) Column-level check constraints are applied only to the column and cannot reference data in another other column

b) Table-level check constraints can reference any column within a table but cannot reference columns in other tables

Default constraints:
Default constraints enable the SQL Server to write default value to a column when user doesn’t specify a value.

Unique constraints:
A unique constraint restricts a column or combination of columns from allowing duplicate values.

Primary key constraints:
Primary key constraints will allow a row to be uniquely identified. This will perform by primary key on the table.

Foreign key constraints:
Foreign keys constraints will ensure that the values that can be entered in a particular column exist in a specified table.