I have doubts over null values.Therefore can you please explain me briefly what is a null value in sql and can we use null values at any place in sql tables?
Columns defined with a PRIMARY KEY constraint or IDENTITY property cannot allow null values.
The nullability of a column determines whether the rows in the table can contain a null value for that column. A null value, or NULL, is different from zero (0), blank, or a zero-length character string such as "". NULL means that no entry has been made. The presence of NULL typically implies that the value is either unknown or undefined.
A value of NULL indicates that the value is unknown. A value of NULL is different from an empty or zero value. No two null values are equal. Comparisons between two null values, or between a NULL and any other value, return unknown because the value of each NULL is unknown.
Null values generally indicate data that is unknown, not applicable, or that the data will be added later. For example, a customer's middle initial may not be known at the time the customer places an order.
SET ANSI_NULLS setting can change the behavior of NULL. In future versions this setting will be ON by default and you will not be able to turn it off. Details here: https://docs.microsoft.com/en-us/sql/t-sql/statements/set-ansi-nulls-transact-sql
This is an article which go in depth about NULL. https://www.simple-talk.com/sql/t-sql-programming/how-to-get-nulls-horribly-wrong-in-sql-server/
Few example of NULL in TSQL here.