Difference between revisions of "The JOIN operation"

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   <h2>JOIN and UEFA EURO 2012</h2>
 
   <h2>JOIN and UEFA EURO 2012</h2>
   <p>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.</p>
+
   <p>This tutorial introduces <code>JOIN</code> 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.</p>
  
 
<div class="progress_panel"><div>
 
<div class="progress_panel"><div>
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<div class='qu'>
 
<div class='qu'>
 
From the previous query you can see that Lars Bender's goal was scored in game 1012.
 
From the previous query you can see that Lars Bender's goal was scored in game 1012.
Notice that the column '''matchid''' in the '''goal''' table corresponds to the '''id''' column in the '''game''' table.
+
Notice that the column <code>matchid</code> in the <code>goal</code> table corresponds to the <code>id</code> column in the <code>game</code> table.
 
<p class='imper'>Show id, stadium, team1, team2 for game 1012</p>
 
<p class='imper'>Show id, stadium, team1, team2 for game 1012</p>
  
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SELECT id,stadium,team1,team2
 
SELECT id,stadium,team1,team2
 
  FROM game
 
  FROM game
WHERE id=1023</source>
+
WHERE id=1012</source>
 
</div>
 
</div>
  
 
<div class='qu'>
 
<div class='qu'>
You can combine the two steps into a single query with a join. You will get all the '''game''' details and all the '''goal''' details if you use
+
You can combine the two steps into a single query with a <code>JOIN</code>. You will get all the <code>game</code> details and all the <code>goal</code> details if you use
 
  SELECT *
 
  SELECT *
 
   FROM game JOIN goal ON (id=matchid)
 
   FROM game JOIN goal ON (id=matchid)
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<div class='qu'>
 
<div class='qu'>
Use the same join as in the previous question.
+
Use the same <code>JOIN</code> as in the previous question.
 
<p class='imper'>Show the team1, team2 and player for every goal scored by a player called Mario <code>player LIKE 'Mario%'</code></p>
 
<p class='imper'>Show the team1, team2 and player for every goal scored by a player called Mario <code>player LIKE 'Mario%'</code></p>
  
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<div class='qu'>
 
<div class='qu'>
The table '''eteam''' gives details of every national team including the coach. You can JOIN '''goal''' to '''eteam''' using the phrase <code>goal JOIN eteam on teamid=id</code>
+
The table <code>eteam</code> gives details of every national team including the coach. You can <code>JOIN</code> <code>goal</code> to <code>eteam</code> using the phrase <code>goal JOIN eteam on teamid=id</code>
<p class='imper'>Show player, teamid, coach, gtime for all goals scored in the first 10 minutes <code>gtime&lt;=10</code></p>
+
<p class='imper'>Show <code>player</code>, <code>teamid</code>, <code>coach</code>, <code>gtime</code> for all goals scored in the first 10 minutes <code>gtime&lt;=10</code></p>
  
 
<source lang='sql' class='def'>
 
<source lang='sql' class='def'>
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<div class='qu'>
 
<div class='qu'>
To JOIN '''game''' with '''eteam''' you could use either<br/> <code>game JOIN eteam ON (team1=eteam.id)</code> or <code>game JOIN eteam ON (team2=eteam.id)</code>
+
To <code>JOIN</code> <code>game</code> with <code>eteam</code> you could use either<br/> <code>game JOIN eteam ON (team1=eteam.id)</code> or <code>game JOIN eteam ON (team2=eteam.id)</code>
  
Notice that because '''id''' is a column name in both '''game''' and '''eteam''' you must specify '''eteam.id''' instead of just '''id'''
+
Notice that because <code>id</code> is a column name in both <code>game</code> and <code>eteam</code> you must specify <code>eteam.id</code> instead of just <code>id</code>
<p class='imper'>List the dates of the matches in which 'Fernando Santos' was the team1 coach.</p>
+
<p class='imper'>List the the dates of the matches and the name of the team in which 'Fernando Santos' was the team1 coach.</p>
  
 
<source lang='sql' class='def'>
 
<source lang='sql' class='def'>
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<div class='qu'>
 
<div class='qu'>
<p class='imper'>List the player for every goal scored in a game where the staium was 'National Stadium, Warsaw'</p>
+
<p class='imper'>List the player for every goal scored in a game where the stadium was 'National Stadium, Warsaw'</p>
  
 
<source lang='sql' class='def'>
 
<source lang='sql' class='def'>
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<p class='imper'>Show names of all players who scored a goal against Germany.</p>
 
<p class='imper'>Show names of all players who scored a goal against Germany.</p>
 
<div class="hint" title="HINT">
 
<div class="hint" title="HINT">
Select goals scored by non-German players in matches where GER was the id of either '''team1''' or '''team2'''.
+
Select goals scored by non-German players in matches where GER was the id of either '''team1''' or '''team2'''. (The "correct result" maybe wrong, because Robin is from NED team and he is not able to play in the GER vs GRE semi-final game.)
  
 
You can use <code>teamid!='GER'</code> to prevent listing German players.
 
You can use <code>teamid!='GER'</code> to prevent listing German players.
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<div class='qu'>
 
<div class='qu'>
<div class='imper'>For every match involving 'POL', show the matchid date and the number of goals scored.</div>
+
<div class='imper'>For every match involving 'POL', show the matchid, date and the number of goals scored.</div>
 
<source lang='sql' class='def'>
 
<source lang='sql' class='def'>
 
SELECT matchid,mdate, team1, team2,teamid
 
SELECT matchid,mdate, team1, team2,teamid
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<div class='qu'>
 
<div class='qu'>
  
<div class='imper'>For every match where 'GER' scored, show the number of goals scored by 'GER'</div>
+
<div class='imper'>For every match where 'GER' scored, show matchid, match date and the number of goals scored by 'GER'</div>
 
<source lang='sql' class='def'>
 
<source lang='sql' class='def'>
 
</source>
 
</source>
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<div class='qu'>
 
<div class='qu'>
  
<div class='imper'>List every match with the goals scored by each team as shown.</div>
+
<div class='imper'>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.</div>
 
<table class="sqlmine">
 
<table class="sqlmine">
 
<tr><th>mdate</th><th>team1</th><th>score1</th><th>team2</th><th>score2</th></tr>
 
<tr><th>mdate</th><th>team1</th><th>score1</th><th>team2</th><th>score2</th></tr>
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<tr><td colspan=5>...</td></tr>
 
<tr><td colspan=5>...</td></tr>
 
</table>
 
</table>
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.
+
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, matchid, team1 and team2.'''
 
<source lang='sql' class='def'>
 
<source lang='sql' class='def'>
SELECT matchid,
+
SELECT mdate,
 
   team1,
 
   team1,
   CASE(WHEN teamid=team1) THEN 1 ELSE 0 END score1,
+
   CASE WHEN teamid=team1 THEN 1 ELSE 0 END score1
  team2
+
   FROM game JOIN goal ON matchid = id
  CASE(WHEN teamid=team2) THEN 1 ELSE 0 END score2,
+
   FROM goal
+
WHERE (teamid='GER')
+
 
</source>
 
</source>
  
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   team2,
 
   team2,
 
   SUM(CASE WHEN teamid=team2 THEN 1 ELSE 0 END) score2
 
   SUM(CASE WHEN teamid=team2 THEN 1 ELSE 0 END) score2
   FROM game JOIN goal ON matchid = id  
+
   FROM game LEFT JOIN goal ON matchid = id  
 
GROUP BY mdate,matchid,team1,team2
 
GROUP BY mdate,matchid,team1,team2
 
</source>
 
</source>

Latest revision as of 23:48, 6 April 2014

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
1001 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.

Summary

The first example shows the goal scored by 'Bender'.

Show matchid and player name for all goals scored by Germany. 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 goal was scored in game 1012. Notice that the column matchid in the goal table corresponds to the id column in the game table.

Show id, stadium, team1, team2 for game 1012

SELECT id,stadium,team1,team2
  FROM game 
 WHERE stadium LIKE '%Warsaw%'
SELECT id,stadium,team1,team2
 FROM game
WHERE id=1012

You can combine the two steps into a single query with a JOIN. You will get all the game details and all the goal details if you use

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

Show the player, teamid and mdate and for every German goal. teamid='GER'

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

This query shows all goals scored in Germany-Greece quarterfinal.

Show names of all players who scored a goal against Germany.

Select goals scored by non-German players in matches where GER was the id of either team1 or team2. (The "correct result" maybe wrong, because Robin is from NED team and he is not able to play in the GER vs GRE semi-final game.)

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.
mdateteam1score1team2score2
1 July 2012ESP4ITA 0
10 June 2012ESP1ITA1
10 June 2012IRL1CRO3
...

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, matchid, team1 and team2.

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,matchid,team1,team2
Clear your results

Old JOIN Tutorial

The next tutorial about the Movie database involves some slightly more complicated joins.

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