Difference between revisions of "The JOIN operation"

Revision as of 13:18, 29 September 2012

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=1023

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 and team2 for every goal scored by a player called Mario player LIKE 'Mario%'</p>


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


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>
<p class='imper'>Show player, the teamid and the coach for all goals scored in the first 10 minutes <code>gtime<=10</code></p>

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


This query shows names of the teams that have scored less than 5 goals in the entire championship.
<p class='imper'>Show player names and count of goals for players who scored more than 2 goals.</p>

SELECT teamname, COUNT(*)
  FROM eteam JOIN goal ON teamid = id
    GROUP BY teamname
      HAVING COUNT(*) < 5
SELECT player, COUNT(*)
  FROM eteam JOIN goal ON teamid = id
    GROUP BY player
      HAVING COUNT(*) > 2


This example uses <code>COUNT()</code>, <code>GROUP BY</code> and <code>SELECT within SELECT</code> to show teams that have scored more goals than England. Add <code>JOIN</code> and...
<p class='imper'>Show stadium and count of goals for stadiums where the number of goals is larger than number of goals scored by Spanish players (teamid = 'ESP') during the entire championship.</p>

SELECT teamid, COUNT(*)
  FROM goal 
    GROUP BY teamid
      HAVING COUNT(*) > 
      (SELECT COUNT(*) 
         FROM goal WHERE teamid = 'ENG')
SELECT stadium, COUNT(*)
  FROM game JOIN goal ON matchid = id
    GROUP BY stadium
      HAVING COUNT(*) >= 
      (SELECT COUNT(*) 
         FROM goal WHERE teamid = 'ESP')









 More difficult questions

This query shows all goals scored in Germany-Greece quarterfinal.
<p class='imper'>Show names of all non-German players who scored a goal in matches against Germany.</p>

Select goals scored by non-German players in matches where GER was the id of either team1 or team2.
SELECT player, gtime
  FROM game JOIN goal ON matchid = id 
    WHERE (team1 = "GER" AND team2 = "GRE")
SELECT player 
  FROM game JOIN goal ON matchid = id 
    WHERE (team1 = "GER" OR team2 = "GER") 
    AND teamid != "GER"


The example shows result of the final match. <code>COUNT()</code> has been used to count the total number of goals.


Show team1, team2 and results of all matches played at 'PGE Arena Gdansk' stadium.
SELECT team1, team2, 
   (SELECT COUNT(*) FROM goal 
     WHERE matchid = id AND teamid = team1) 
   AS team1, 
   (SELECT COUNT(*) FROM goal 
     WHERE matchid = id AND teamid = team2) 
   AS team2 
  FROM game WHERE id = 1031
SELECT team1, team2, 
   (SELECT COUNT(*) FROM goal 
     WHERE matchid = id AND teamid = team1) 
   AS team1, 
   (SELECT COUNT(*) FROM goal 
     WHERE matchid = id AND teamid = team2) 
   AS team2 
  FROM game WHERE stadium = 'PGE Arena Gdansk'





The following query shows matches which were won by team1.

Show id of matches in which team1 scored at least two more goals than team2.

SELECT team1, team2 FROM game WHERE 
   (SELECT COUNT(*) FROM goal 
     WHERE matchid = id AND teamid = team1) >
   (SELECT COUNT(*) FROM goal 
     WHERE matchid = id AND teamid = team2)
SELECT id FROM game WHERE 
   (SELECT COUNT(*) FROM goal 
     WHERE matchid = id AND teamid = team1) -
   (SELECT COUNT(*) FROM goal 
     WHERE matchid = id AND teamid = team2) >= 2


The example shows teams that scored a goal at stadiums where Poland has played its matches.

Show players that scored a goal at 'National Stadium, Warsaw' in a day (mdate) when Czech Republic ('CZE') didn't play a match. Use <code>DISTINCT</code> to remove duplicates from your query results.
SELECT DISTINCT teamid 
  FROM game JOIN goal ON matchid = id 
    WHERE Stadium IN 
        (SELECT stadium FROM game 
           WHERE team1 = 'POL' OR team2 = 'POL')
SELECT DISTINCT player 
  FROM game JOIN goal ON matchid = id 
    WHERE stadium = 'National Stadium, Warsaw'
      AND mdate NOT IN 
        (SELECT mdate FROM game 
           WHERE team1 = 'CZE' OR team2 = 'CZE')


The query shows the stadiums, where the teams who scored more than 8 goals played their matches.

Show teamid and count of goals for all 8 teams that have played more than 3 matches.
SELECT teamid, COUNT(*) FROM goal 
  WHERE 3 < 
    (SELECT COUNT(DISTINCT id) FROM game 
       WHERE team1 = teamid OR team2 = teamid) 
  GROUP BY teamid
SELECT DISTINCT stadium 
  FROM game WHERE team1 IN
    (SELECT teamid FROM goal 
       GROUP BY teamid HAVING COUNT(*) > 8)


Clear your results
<p>JOIN Quiz</p>

Old JOIN Tutorial <p>The next tutorial about the Movie database involves some slightly more complicated joins.</p>

@@ Line 180: / Line 180: @@
 <div class='qu'>
 Use the same join as in the previous question.
-<p class='imper'>Show the team1 and team2 for every goal scored by 'Alan Dzagoev'</p>
+<p class='imper'>Show the team1 and team2 for every goal scored by a player called Mario <code>player LIKE 'Mario%'</p>
 <source lang='sql' class='def'>
@@ Line 189: / Line 189: @@
 SELECT team1, team2
    FROM game JOIN goal ON (id=matchid)
-WHERE player= 'Alan Dzagoev'</source>
+WHERE player LIKE 'Mario%'</source>
 </div>
 <div class='qu'>
-This example is using JOIN to display all goals scored during the UEFA EURO 2012 final (it was the only match in July).
+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>
+<p class='imper'>Show player, the teamid and the coach for all goals scored in the first 10 minutes <code>gtime&lt;=10</code></p>
-<p class='imper'>Show all goals scored in Germany - Greece match. Use team1 and team2 values to find the match.</p>
 <source lang='sql' class='def'>
-SELECT player, gtime FROM goal
+SELECT player, teamid
-  JOIN game ON id = matchid
+  FROM goal
-    WHERE mdate LIKE '%July%'
+ WHERE gtime&lt;=10
 </source>
 <source lang='sql' class='ans'>
-SELECT player, gtime FROM goal
+SELECT player, teamid, coach
-  JOIN game ON id = matchid
+  FROM goal JOIN eteam ON (teamid=id)
-    WHERE team1 = 'GER' AND team2 = 'GRE'
+ WHERE gtime&lt;=10
 </source>
 </div>

Difference between revisions of "The JOIN operation"

Revision as of 13:18, 29 September 2012

JOIN and UEFA EURO 2012

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