The JOIN operation

game

id	mdate	stadium	team1	team2
1001	8 June 2012	National Stadium, Warsaw	POL	GRE
1002	8 June 2012	Stadion Miejski (Wroclaw)	RUS	CZE
1003	12 June 2012	Stadion Miejski (Wroclaw)	GRE	CZE
1004	12 June 2012	National Stadium, Warsaw	POL	RUS
...

goal

matchid	teamid	player	gtime
1001	POL	Robert Lewandowski	17
1001	GRE	Dimitris Salpingidis	51
1002	RUS	Alan Dzagoev	15
1002	RUS	Roman Pavlyuchenko	82
...

eteam

id	teamname	coach
POL	Poland	Franciszek Smuda
RUS	Russia	Dick Advocaat
CZE	Czech Republic	Michal Bilek
GRE	Greece	Fernando Santos
...

JOIN and UEFA EURO 2012

This tutorial introduces JOIN which allows you to use data from two or more tables. The tables contain all matches and goals from UEFA EURO 2012 Football Championship in Poland and Ukraine.

The data is available (mysql format) at http://sqlzoo.net/euro2012.sql

Summary

The first example shows the goal scored by a player with the last name 'Bender'. The * says to list all the columns in the table - a shorter way of saying matchid, teamid, player, gtime

Modify it to show the matchid and player name for all goals scored by Germany. To identify German players, check for: teamid = 'GER'

SELECT * FROM goal 
  WHERE player LIKE '%Bender'

SELECT matchid, player 
 FROM goal
   WHERE teamid LIKE 'GER'

From the previous query you can see that Lars Bender's scored a goal in game 1012. Now we want to know what teams were playing in that match.

Notice in the that the column matchid in the goal table corresponds to the id column in the game table. We can look up information about game 1012 by finding that row in the game table.

Show id, stadium, team1, team2 for just game 1012

SELECT id,stadium,team1,team2
  FROM game

SELECT id,stadium,team1,team2
 FROM game
WHERE id=1012

You can combine the two steps into a single query with a JOIN.

SELECT *
  FROM game JOIN goal ON (id=matchid)

The FROM clause says to merge data from the goal table with that from the game table. The ON says how to figure out which rows in game go with which rows in goal - the matchid from goal must match id from game. (If we wanted to be more clear/specific we could say
ON (game.id=goal.matchid)

The code below shows the player (from the goal) and stadium name (from the game table) for every goal scored.

Modify it to show the player, teamid, stadium and mdate for every German goal.

SELECT player,stadium
  FROM game JOIN goal ON (id=matchid)

SELECT player,teamid,stadium,mdate
  FROM game JOIN goal ON (id=matchid)
 WHERE teamid='GER'

Use the same JOIN as in the previous question.

Show the team1, team2 and player for every goal scored by a player called Mario player LIKE 'Mario%'

SELECT team1, team2, player
  FROM game JOIN goal ON (id=matchid)
WHERE player LIKE 'Mario%'

The table eteam gives details of every national team including the coach. You can JOIN goal to eteam using the phrase goal JOIN eteam on teamid=id

Show player, teamid, coach, gtime for all goals scored in the first 10 minutes gtime<=10

SELECT player, teamid, gtime
  FROM goal 
 WHERE gtime<=10

SELECT player, teamid, coach, gtime
  FROM goal JOIN eteam ON (teamid=id)
 WHERE gtime<=10

To JOIN game with eteam you could use either
game JOIN eteam ON (team1=eteam.id) or game JOIN eteam ON (team2=eteam.id)

Notice that because id is a column name in both game and eteam you must specify eteam.id instead of just id

List the dates of the matches and the name of the team in which 'Fernando Santos' was the team1 coach.

SELECT mdate,teamname
  FROM game JOIN eteam ON (team1=eteam.id)
WHERE coach='Fernando Santos'

List the player for every goal scored in a game where the stadium was 'National Stadium, Warsaw'

SELECT player
  FROM goal JOIN game ON (id=matchid)
 WHERE stadium = 'National Stadium, Warsaw'

More difficult questions

The example query shows all goals scored in the Germany-Greece quarterfinal.

Instead show the name of all players who scored a goal against Germany.

Select goals scored only by non-German players in matches where GER was the id of either team1 or team2.

You can use teamid!='GER' to prevent listing German players.

You can use DISTINCT to stop players being listed twice.

SELECT player, gtime
  FROM game JOIN goal ON matchid = id 
    WHERE (team1='GER' AND team2='GRE')

SELECT DISTINCT player
  FROM game JOIN goal ON matchid = id 
 WHERE (team1 = 'GER' OR team2 = 'GER')
   AND teamid!='GER'

Show teamname and the total number of goals scored.

You should COUNT(*) in the SELECT line and GROUP BY teamname

SELECT teamname, player
  FROM eteam JOIN goal ON id=teamid
 ORDER BY teamname

SELECT teamname,COUNT(teamid)
  FROM eteam JOIN goal ON id=teamid
GROUP BY teamname

Show the stadium and the number of goals scored in each stadium.

SELECT stadium,COUNT(1)
  FROM goal JOIN game ON id=matchid
GROUP BY stadium

For every match involving 'POL', show the matchid, date and the number of goals scored.

SELECT matchid,mdate, team1, team2,teamid
  FROM game JOIN goal ON matchid = id 
 WHERE (team1 = 'POL' OR team2 = 'POL')

SELECT matchid,mdate,COUNT(teamid)
  FROM game JOIN goal ON matchid = id 
 WHERE (team1 = 'POL' OR team2 = 'POL')
GROUP BY matchid,mdate

For every match where 'GER' scored, show matchid, match date and the number of goals scored by 'GER'

SELECT matchid,mdate,COUNT(teamid)
  FROM game JOIN goal ON matchid = id 
 WHERE (teamid='GER')
GROUP BY matchid,mdate

List every match with the goals scored by each team as shown. This will use "CASE WHEN" which has not been explained in any previous exercises.

mdate	team1	score1	team2	score2
1 July 2012	ESP	4	ITA	0
10 June 2012	ESP	1	ITA	1
10 June 2012	IRL	1	CRO	3
...

Notice in the query given every goal is listed. If it was a team1 goal then a 1 appears in score1, otherwise there is a 0. You could SUM this column to get a count of the goals scored by team1. Sort your result by mdate, team1 and team2.

Some games have no goals scored by either side, so a regular JOIN query will exclude them because they will not have related entries on the goal table.
Look up how LEFT JOIN works.

SELECT mdate,
  team1,
  CASE WHEN teamid=team1 THEN 1 ELSE 0 END score1
  FROM game JOIN goal ON matchid = id

SELECT mdate,
  team1,
  SUM(CASE WHEN teamid=team1 THEN 1 ELSE 0 END) score1,
  team2,
  SUM(CASE WHEN teamid=team2 THEN 1 ELSE 0 END) score2
  FROM game LEFT JOIN goal ON matchid = id 
GROUP BY mdate,team1,team2

What next?

JOIN Quiz

Old JOIN Tutorial

More JOIN operations: The next tutorial about the Movie database involves some slightly more complicated joins from the movie database.