Correct Answer: A Subquery or Inner query or a Nested query is a query within another SQL query and embedded within the WHERE clause Subqueries are most frequently used with the SELECT statement The basic syntax is as follows:: SELECT column_name [, column_name ] FROM table1 [, table2 ] WHERE column_name OPERATOR (SELECT column_name [, column_name ] FROM table1 [, table2 ] [WHERE]) A subquery is used to return data that will be used in the main query as a condition to further restrict the data to be retrieved Subqueries can be used with the SELECT, INSERT, UPDATE, and DELETE statements along with the operators like =, <, >, >=, <=, IN, BETWEEN, etc There are a few rules that subqueries must follow − 1 Subqueries must be enclosed within parentheses 2 A subquery can have only one column in the SELECT clause, unless multiple columns are in the main query for the subquery to compare its selected columns 3 An ORDER BY command cannot be used in a subquery, although the main query can use an ORDER BY The GROUP BY command can be used to perform the same function as the ORDER BY in a subquery 4 Subqueries that return more than one row can only be used with multiple value operators such as the IN operator 5 The SELECT list cannot include any references to values that evaluate to a BLOB, ARRAY, CLOB, or NCLOB 6 A subquery cannot be immediately enclosed in a set function 7 The BETWEEN operator cannot be used with a subquery However, the BETWEEN operator can be used within the subquery
