Using subqueries

We often use subqueries in our SQL statements to nest more queries in one statement, using the results from an "inner" query to calculate other values.

In this recipe, we will see the use of subqueries for getting only a subset of records, demonstrating the constructs (NOT) EXISTS and (NOT) IN, highlighting the semantic difference between them (and when to choose one type of statement or the other).

How to do it...

The following steps will demonstrate the use of subqueries:

1. Connect to the SH schema:

   CONNECT sh@TESTDB/sh
   SET AUTOT TRACE EXP STAT
   

2. Select a table using the IN operator:

   SET AUTOT TRACE EXP STAT
   SELECT AMOUNT_SOLD FROM sh.SALES S
   WHERE S.CUST_ID IN (
     SELECT C.CUST_ID FROM sh.CUSTOMERS C
     WHERE C.CUST_CREDIT_LIMIT IN (10000, 11000, 15000)
   );
   

3. Rewrite the same query using the EXISTS construct:

   SELECT AMOUNT_SOLD FROM sh.SALES S
   WHERE EXISTS (
     SELECT NULL FROM sh.CUSTOMERS C
     WHERE S.CUST_ID = C.CUST_ID
     AND C.CUST_CREDIT_LIMIT IN (10000, 11000, 15000)
   );
   

4. Select...