SQLZoo刷题系列 5
The JOIN operation
刷题网站SQLZoo，刷题语言MySQL
知识
在这个练习中，需要使用 JOIN 的知识， JOIN 用于根据两个或多个表中的列之间的关系，从这些表中查询数据。
可以查看该篇blog --> MySQL的JOIN（一）：用法
题目
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 |
| … |
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
1.
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 matchid, player FROM goal
WHERE teamid = 'GER'
2.
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
WHERE id = 1012
3.
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, teamid,stadium, mdate
FROM game INNER JOIN goal ON (teamid='GER') and (game.id=goal.matchid)
注：记得要让 game.id=goal.matchid，去除大量无用项
4.
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 INNER JOIN goal ON (player LIKE 'Mario%') and (game.id=goal.matchid)
5.
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, coach, gtime
FROM goal INNER JOIN eteam ON (goal.teamid=eteam.id)
WHERE gtime<=10
6.
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 game.mdate,eteam.teamname
FROM game INNER join eteam on (eteam.id=game.team1) and (eteam.coach='Fernando Santos')
7.
List the player for every goal scored in a game where the stadium was ‘National Stadium, Warsaw’
SELECT goal.player
from goal INNER JOIN game on (game.stadium='National Stadium, Warsaw') and (game.id=goal.matchid)
8.
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 DISTINCT(player)
from goal INNER JOIN game on (game.id = goal.matchid) and (goal.teamid!='GER') and (game.team1='GER' or game.team2='GER')
9.
Show teamname and the total number of goals scored.
SELECT teamname, COUNT(teamid)
FROM eteam INNER JOIN goal ON id=teamid
GROUP BY teamname
注：count（）等函数要配合group by使用
10.
Show the stadium and the number of goals scored in each stadium.
select stadium, count(1)
from game INNER JOIN goal on id=matchid
group by stadium
11.
For every match involving ‘POL’, show the matchid, date and the number of goals scored.
SELECT matchid, mdate, count(teamid)
FROM game INNER JOIN goal ON matchid = id
WHERE (team1 = 'POL' OR team2 = 'POL')
group by matchid, mdate
12.
For every match where ‘GER’ scored, show matchid, match date and the number of goals scored by ‘GER’
select matchid, mdate, count(1)
from goal INNER JOIN game on matchid=id
where teamid='GER'
group by matchid, mdate
13.
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, matchid, team1 and team2.
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