Know about DBMS

Null value: SQL allows the use of null values to indicate absence of information about the value of an attribute.
Sub queries: The purpose of subquery is to drive a list of information from which a query can choose appropriate rows.
Set Comparison: This is a test that uses comparison operators to compare values in the temporary table with the values in the table used by the query.
Set membership: This is similar to existence test. It determines if a value in the current row of the table used by the query also exists in the temporary table.