sqlzoo - The JOIN operation/zh 答案

参考链接

1. 第一個例子列出球員姓氏為’Bender’的入球數據。 * 表示列出表格的全部欄位，簡化了寫matchid, teamid, player, gtime語句。

修改此SQL以列出 賽事編號matchid 和球員名 player ,該球員代表德國隊Germany入球的。要找出德國隊球員，要檢查: teamid = ‘GER’

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

2. 由以上查詢，你可見Lars Bender’s 於賽事 1012入球。.現在我們想知道此賽事的對賽隊伍是哪一隊。

留意在 goal 表格中的欄位 matchid ，是對應表格game的欄位id。我們可以在表格 game中找出賽事1012的資料。
只顯示賽事1012的 id, stadium, team1, team2

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

3. 我們可以利用JOIN來同時進行以上兩個步驟。

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

語句FROM 表示合拼兩個表格game 和 goal的數據。語句 ON 表示如何找出 game中每一列應該配對goal中的哪一列 – goal的 id 必須配對game的 matchid 。 簡單來說，就是 ON (game.id=goal.matchid)
以下SQL列出每個入球的球員(來自goal表格)和場館名(來自game表格)
修改它來顯示每一個德國入球的球員名，隊伍名，場館和日期。

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

4. 使用上題相同的 JOIN語句，列出球員名字叫Mario (player LIKE ‘Mario%’)有入球的 隊伍1 team1, 隊伍2 team2 和 球員名 player

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

5. 表格eteam 貯存了每一國家隊的資料，包括教練。你可以使用語句 goal JOIN eteam on teamid=id來合拼 JOIN 表格goal 到 表格eteam。列出每場球賽中首10分鐘gtime<=10有入球的球員 player, 隊伍teamid, 教練coach, 入球時間gtime

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

6. 要合拼JOIN 表格game 和表格 eteam，你可以使用
   game JOIN eteam ON (team1=eteam.id)
   或
   game JOIN eteam ON (team2=eteam.id)
   注意欄位id同時是表格game 和表格 eteam的欄位，你要清楚指出eteam.id而不是只用id
   列出’Fernando Santos’作為隊伍1 team1 的教練的賽事日期，和隊伍名。

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

7. 列出場館 'National Stadium, Warsaw’的入球球員。

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

8. 以下例子找出德國-希臘Germany-Greece 的八強賽事的入球
   修改它，只列出全部賽事，射入德國龍門的球員名字。
   找非德國球員的入球，德國可以在賽事中作team1 隊伍１（主）或team2隊伍２（客）。 你可以用teamid!=‘GER’ 來防止列出德國球員。 你可以用DISTINCT來防止球員出現兩次以上。

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

9. 列出隊伍名稱 teamname 和該隊入球總數
   COUNT and GROUP BY
   你應該在SELECT語句中使用COUNT(*)和使用GROUP BY teamname

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

10. 列出場館名和在該場館的入球數字。

SELECT stadium, COUNT(player)
FROM game ga JOIN goal go ON (ga.id= go.matchid)
GROUP BY stadium

11. 每一場波蘭’POL’有參與的賽事中，列出賽事編號 matchid, 日期date 和入球數字。

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

疑问：在使用 GROUP BY + COUNT(teamid) 的时候，需要指定所有的参数

12. 每一場德國’GER’有參與的賽事中，列出賽事編號 matchid, 日期date 和德國的入球數字。

SELECT matchid, mdate, COUNT(teamid)
FROM game ga JOIN goal go ON (ga.id= go.matchid)
WHERE (ga.team1 = 'GER' OR ga.team2 = 'GER') AND 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.
    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 ga LEFT OUTER JOIN goal go ON (ga.id= go.matchid)
GROUP BY mdate, team1, team2

疑问：这里为什么需要用到 LEFT OUTER JOIN