Combinations
A JOIN with no join conditions results in every row in one table being connected to every row in another table, forming all possible row combinations.
Often this is done by mistake, but it can be useful. In this example we use this as an advantage to show the goals made for all games.
DROP TABLE teams;
DROP TABLE tscores
CREATE TABLE teams(
teamname VARCHAR(20));
INSERT INTO teams VALUES ('Lions');
INSERT INTO teams VALUES ('Tigers');
INSERT INTO teams VALUES ('Wildcats');
INSERT INTO teams VALUES ('Toads');
CREATE TABLE tscores (
hometeam VARCHAR (20),
awayteam VARCHAR (20),
homescore INTEGER,
awayscore INTEGER );
INSERT INTO tscores VALUES ('Lions', 'Wildcats', 1, 4);
INSERT INTO tscores VALUES ('Toads', 'Tigers', 3, 5);
INSERT INTO tscores VALUES ('Wildcats', 'Tigers', 0, 0);
INSERT INTO tscores VALUES ('Lions', 'Toads', 0, 0);
INSERT INTO tscores VALUES ('Wildcats', 'Toads', 0, 0);
INSERT INTO tscores VALUES ('Lions', 'Tigers', 0, 0);
INSERT INTO tscores VALUES ('Tigers', 'Lions', 0, 0);
INSERT INTO tscores VALUES ('Toads', 'Wildcats', 0, 0);
INSERT INTO tscores VALUES ('Toads', 'Lions', 0, 0);
INSERT INTO tscores VALUES ('Tigers', 'Wildcats', 0, 0);
INSERT INTO tscores VALUES ('Tigers', 'Toads', 0, 0);
INSERT INTO tscores VALUES ('Wildcats', 'Lions', 0, 0);
To get all possible values a CROSS JOIN is used.
Conditions as well as a LEFT JOIN are then used to filter the query and make it more accurate.
SELECT home.teamname Home, away.teamname Away,
tscores.homescore, tscores.awayscore
FROM teams home CROSS JOIN teams away LEFT JOIN tscores ON
(home.teamname = tscores.hometeam
AND tscores.awayteam = away.teamname)
WHERE home.teamname != away.teamname
Hack 10 Converting subqueries into joins
Hack 11 Converting aggregate subqueries into joins
Hack 16 Search for a String across columns
Hack 24 Multiply Across a Result Set
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 72 Extract a subset of the results
Hack 78 Break it down by Range
Hack 88 Test two values from a subquery
