Test subquery

Here you are shown how to test two values from your subquery

to ensure that it has run correctly.

Table 1

Customer	Item	Price
Brian	Table	100
Robert	Chair	20
Robert	Carpet	200
Janette	Statue	300

Table 2

Customer	Item	Price
Brian	Table	100
Robert	Carpet	200
Janette	Statue	300

schema:scott

DROP TABLE custItem;

CREATE TABLE custItem (
   Customer VARCHAR(20),
   Item VARCHAR(20),
   Price INT );
INSERT INTO custItem VALUES ('Brian','Table',100);
INSERT INTO custItem VALUES ('Robert','Chair',20);
INSERT INTO custItem VALUES ('Robert','Carpet',200);
INSERT INTO custItem VALUES ('Janette','Statue',300);

Suppose you have a table of customers and their orders, as shown in Table 1 and you want to produce a list of every customer and their biggest order, as shown in Table 2. This is easy enough to do with:

SELECT Customer, MAX(price)
 FROM custItem
 GROUP BY Customer

But by testing the results you can obtain the item that was purchased as well.

SELECT x.Customer, x.Item, x.Price
  FROM custItem x JOIN (
      SELECT Customer, MAX(price) AS Price
        FROM custItem
        GROUP BY Customer) y
    ON (x.Customer = y.Customer AND x.Price = y.Price)

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