What is a Sub Query? Describe its Types?

A sub query is a SELECT statement that is embedded in a clause of another SELECT statement. Sub query can be placed in WHERE, HAVING and FROM clause.

Guidelines for using sub queries:

1. Enclose sub queries within parenthesis
2. Place sub queries on the right side of the comparison condition.
3. Use Single-row operators with single-row sub queries and Multiple-row operators with multiple-row sub queries.

Types of sub queries:

1. Single-Row Sub query: Queries that return only one row from the Inner select statement. Single-row comparison operators are: =, >, >=, <, <=, <>
2. 
   
3. Multiple-Row Sub query: Queries that return more than one row from the inner Select statement. There are also multiple-column sub queries that return more than one column from the inner select statement. Operators includes: IN, ANY, ALL.