MySQL的簡單連接

Question

我想執行一個簡單的MySQL的加入：

我有matches其特點，我想加入的兩個字段的表：team1和team2。

我想找到存儲在該teams檯球隊相應名稱：

SELECT teams.team_name AS "name1", teams.team_name AS "name2", matches.id 
FROM teams, matches 
WHERE matches.id=1 
AND matches.team1_id=teams.team_id 
AND matches.team2_id=teams.team_id 

如果我刪除最後和條件，我得到一個結果，但無論如果我包括我得到一個空集？

我在做什麼錯？

Answer 1

對於您希望在查詢中唯一引用的每個團隊，您需要一個單獨的別名，否則SQL將比較同一行。

SELECT team1.team_name AS "name1", team2.team_name AS "name2", matches.id 
FROM teams team1, teams team2, matches 
WHERE matches.id=1 
AND matches.team1_id=team1.team_id 
AND matches.team2_id=team2.team_id 

在這裏，我們現在有兩個team1和team2的團隊表別名，因此他們可以分別引用不同的行。

Answer 2

你需要加入teams兩次表matches，因爲兩列都依賴於它，

SELECT a.*,    -- <<== select column that you want to project 
     b.team_name AS Team1Name, 
     c.Team_name AS Team2Name 
FROM matches a 
     INNER JOIN teams b 
      ON a.team1_ID = b.team_ID 
     INNER JOIN teams c 
      ON a.team2_ID = c.team_ID 
-- WHERE a.id = 1 

爲了進一步獲得更多的知識有關加入，請訪問以下鏈接：

• Visual Representation of SQL Joins

Answer 3

您的查詢原樣是隻會返回小組播放的記錄反對自己（當然從未發生過）。你希望兩個不同的加入對球隊，所以它的兩個實例：

select 
    teams1.team_name as "name1" 
    ,teams2.team_name as "name2" 
from 
    matches 
join 
    teams teams1 on matches.team1_id = teams1.team_id 
join 
    teams teams2 on matches.team2_id = teams2.team_id 
where 
    matches.id = 1 

Answer 4

試試這個：

SELECT team1.team_name AS "name1", team2.team_name AS "name2", matches.id 
FROM teams as team1, teams as team2, matches 
WHERE matches.id=1 
AND matches.team1_id=team1.team_id 
AND matches.team2_id=team2.team_id