Forgotten rows

In this example you are shown how to include the rows your JOIN function automatically leaves out as it does not perform the join when one of the values is 0.

This is done by adding a either a LEFT JOIN or a UNION making the join also to reveal the rows with a count of 0.

Table 1 shows the results without a LEFT JOIN and table 2 shows the results we obtain with the LEFT JOIN

Table 1

name	COUNT(custid)
Betty	2
Janette	1

Table 2

name	COUNT(custid)
Betty	2
Janette	1
Robert	0

schema:scott

DROP TABLE customer;
DROP TABLE invoice;

 CREATE TABLE customer(
  id INTEGER,
  name VARCHAR(20));
INSERT INTO customer VALUES (1,'Betty');
INSERT INTO customer VALUES (2,'Robert');
INSERT INTO customer VALUES (3,'Janette');
CREATE TABLE invoice(
  invoiceno INTEGER,
  whn DATE,
  custid INTEGER,
  cost INTEGER );
INSERT INTO invoice VALUES (1,'2006-11-01',1,100);
INSERT INTO invoice VALUES (2,'2006-11-05',1,500);
INSERT INTO invoice VALUES (3,'2006-11-11',3,200);

The following query will only give two rows as the JOIN function automatically does not include rows with a count of 0.

SELECT name, COUNT(custid)
  FROM customer JOIN invoice ON (id=custid)
  GROUP BY name

In order to obtain the rows where the count from the query is 0 a LEFT JOIN or a UNION can be used.

 
SELECT name, COUNT(custid)
  FROM customer LEFT JOIN invoice ON (id=custid)
  GROUP BY name

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