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.

schema:scott

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

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