7. Using Sub Queries

2. USING SUB-QUERIES, TO SOLVE QUERIES.

3. OBJECTIVES
At the end of this module, students should be able
to:
1. Define sub-queries.
2. Know type of problems that sub-queries
can solve.
3. List the types of sub-queries
4. Write single row and multiple row sub-
queries.

4. SUB-QUERIES
 A sub-query –also called a nested SELECT
statement- is defined as a query, within
another query.
 It may appear in the WHERE clause, in the HAVING
clause, or in the FROM clause.
 When placing a restriction on a query block,
we always specify the conditions for the
restriction in the WHERE clause.

5. EXAMPLE:

6.  Under certain conditions, you may need to
write a query to determine the WHERE condition.

7. EXERCISE:
 Using a single SELECT statement, query the
employees table for those employees who
earn a salary below Nancy’s salary.
 However, if you want to query for the details
of all the staff who earn a salary less than that
of Nancy you would use a sub-query for
Nancy’s salary, and the result in the WHERE
condition of our SELECT statement.

8.  We have two types of sub-queries. The single
row sub-queries and the multiple row sub-
queries.

9. SINGLE ROW SUB_QUERIES
 The single row sub-queries, would return only
one value, from the inner SELECT statement. In a
single row sub-query, the outer query,
contains a single row comparison operator,
like an equal to, equal to or greater than, or
less than.

10. EXAMPLE:

11.  This is a single row sub-query because the
inner SELECT statement will return only one value
and the outer SELECT statement uses a single
comparison operator to pick up that value.

12. MULTIPLE ROW SUB_QUERIES
 A multiple row sub-query on the contrary
would return more than one row from the
inner SELECT statement.
 The outer SELECT will therefore need a multiple
comparison operator like ANY, IN or ALL to pick the
multiple values returned by the inner query.

13. EXAMPLE:

14.  The Oracle server first takes the inner
query and searches for the DISTINCT values in
the salary of employees, with STORE CLERK as the
job_id.
 It then places this numbers in the bracket
next to the ANY keyword. And that is what
gives us the result displayed, on the
previous screen.