Difference between revisions of "The JOIN operation"
|Line 1:||Line 1:|
Revision as of 22:48, 26 September 2016
|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|
|CZE||Czech Republic||Michal Bilek|
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 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
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 id from goal must match matchid from game. (If we wanted to be more clear/specific we could say
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 and 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%'
eteam gives details of every national team including the coach. You can
eteam using the phrase
goal JOIN eteam on teamid=id
gtime for all goals scored in the first 10 minutes
SELECT player, teamid, gtime FROM goal WHERE gtime<=10
SELECT player, teamid, coach, gtime FROM goal JOIN eteam ON (teamid=id) WHERE gtime<=10
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
eteam you must specify
eteam.id instead of just
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
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'
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
SELECT stadium,COUNT(1) FROM goal JOIN game ON id=matchid GROUP BY stadium
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
SELECT matchid,mdate,COUNT(teamid) FROM game JOIN goal ON matchid = id WHERE (teamid='GER') GROUP BY matchid,mdate
|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, 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