Forgotten rows

Include the rows your JOIN forgot.

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(*)
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(*)
 FROM customer LEFT JOIN invoice ON (id=custid)

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