Subquery and JOIN2

In this example you are shown how to convert subqueries containing aggregate functions into JOINs allowing for a more efficient response time from the query.

schema:scott

DROP TABLE orders

 CREATE TABLE orders(
  customer VARCHAR(40),
  whn VARCHAR(40),
  totalitems INTEGER );
INSERT INTO orders VALUES ('Jim','2006-10-10', 5);
INSERT INTO orders VALUES ('Jim','2006-10-11', 3);
INSERT INTO orders VALUES ('Jim','2006-10-12', 1);
INSERT INTO orders VALUES ('Brian','2006-10-10', 7);

SELECT customer, whn, totalitems
FROM orders a
WHERE a.whn = (
 SELECT MAX(totalitems)
 FROM orders b
 WHERE a.customer = b.customer)

To make this query more efficient a HAVING clause can be used with a self join to replace the subquery.

SELECT a.customer, a.whn, a.totalitems
  FROM orders a JOIN orders b ON (a.customer = b.customer)
  GROUP BY a.customer, a.whn, a.totalitems
  HAVING a.whn = MAX(b.whn)

SQL HACKS

Hack 10 Converting subqueries into joins

Hack 11 Converting aggregate subqueries into joins

Hack 14 Generate Combinations

Hack 16 Search for a String across columns

Hack 24 Multiply Across a Result Set

Hack 25 Keep a running total

Hack 25.5 Splitting and combining columns

Hack 26 Include the rows your JOIN forgot

Hack 30 Calculate the maximum/minimum of two fields

Hack 33 Get values and subtotals in one shot

Hack 50 Combine tables containing different data

Hack 51/52 Display rows as columns

Hack 55 Import Someone Else's Data

Hack 62 Issue Queries Without Using a Table

Hack 63 Generate rows without tables

Hack 70 Combine your queries

Hack 72 Extract a subset of the results

Hack 78 Break it down by Range

Hack 88 Test two values from a subquery

Hack 98 Find and stop long running queries

Hack 100 Run SQL from a Web Page