Rules to create SQL Sub Queries
A Subquery or Inner query or Nested query is a query within
another SQL query and embedded within the WHERE clause.
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:
- Subqueries must be enclosed within parentheses.
- 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.
- The BETWEEN operator cannot be used with a subquery; however, the BETWEEN operator can be used within the subquery.
- Subqueries that return more than one row can only be used with multiple value operators, such as the IN operator.
- The SELECT list cannot include any references to values that evaluate to a BLOB, ARRAY, CLOB, or NCLOB.
- A subquery cannot be immediately enclosed in a set function.
- An ORDER BY cannot be used in a subquery, although the main query can use an ORDER BY. The GROUP BY can be used to perform the same function as the ORDER BY in a subquery.
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