Nested Queries

Let us see some more complex queries in this section.

Sub-queries:

Some of the basic concerns of sub-queries are:

• A sub-query is a SELECT statement that is fixed in a clause of another SELECT statement. They are often referred to as a SUB SELECT or NESTED SELECT or INNER SELECT.
• The sub-query (inner query) implements first before the main query. The product of the sub-query is used by the outer query (main query).
• Sub-query can be placed in HAVING or WHERE or FROM clauses.
• Format of using sub-queries:

                 SELECT

                 FROM

                 WHERE expr OPERATOR

                                                          (SELECT FROM

                Operator has a comparison operator (one or multiple row operators)

Single row operator:   >, =, >=, <, <=, <>

                         Multiple row operators: IN, ALL, ANY,

• Order by clause can't be used in sub-query, if specified it must be the end clause in the main select statement.
• Parts of sub-queries:

         ‰  Single row sub-query: It gives only single row from the inner select statement.

        ‰   Multiple row sub-queries: It gives more than single row from the inner select statement

        ‰  Multiple column sub-queries: It gives more than single column from the inner select statement.

Single row operators are used with one row sub queries and multiple row operators are used with numerous row sub queries.

• The Inner and Outer queries can get data from different tables.
• Group Functions are used in sub queries.