Question

我有5个表，我正在运行一个SELECT语句，当所有表中都有数据时它一切正常，但如果只有一个表中有数据，我得到零结果？当不是所有表都填充数据时，如何使查询工作。现在正好刮我的头，请帮忙！

SELECT admins.username, round1.player, round1.round1_score, 
     round2.player, round2.round2_score,
     round3.player, round3.round3_score, 
     round4.player, round4.round4_score, 
     round5.player, round5.round5_score 
FROM `admins`, `round1`, `round2`, `round3`, `round4`, `round5` 
WHERE admins.username <> 'admin' 
     AND admins.username = round1.player 
     AND admins.username = round2.player 
     AND admins.username = round3.player 
     AND admins.username = round4.player 
     AND admins.username = round5.player 
GROUP BY admins.username

Answer 1

我假设admins表是主表？您需要使用outer joins代替：

SELECT a.username, r1.player, r1.round1_score, 
       r2.player, r2.round2_score, r3.player, r3.round3_score, 
       r4.player, r4.round4_score, r5.player, r5.round5_score 
FROM `admins` a
    LEFT JOIN `round1` r1 ON a.username = r1.player
    LEFT JOIN `round2` r2 ON a.username = r2.player
    LEFT JOIN `round3` r3 ON a.username = r3.player
    LEFT JOIN `round4` r4 ON a.username = r4.player
    LEFT JOIN `round5` r5 ON a.username = r5.player,
WHERE a.username <> 'admin' 
GROUP BY a.username

我不确定你真的需要group by，一般情况下它用于聚合（有时用于获取distinct个记录）。

Answer 2

The way your query is, is like using inner join so basically if one table has no data you get 0 results. You need to change your query using left join.

Answer 3

尝试内连接或交叉连接表以获得答案我认为它会起作用您可以阅读更多here或here

Answer 4

您正在连接5个表

admins，round1，round2，round3，round4，round5

这是一个内部联接，你只有共同数据

如果您想要数据，即使您右侧的表格为空，也可以使用LEFT JOIN

 SELECT admins.username, round1.player, round1.round1_score, 
 round2.player, round2.round2_score,
 round3.player, round3.round3_score, 
 round4.player, round4.round4_score, 
 round5.player, round5.round5_scoreFROM (((((`admins` left join `round1` on admins.username = round1.player) 
    left join `round2` on admins.username = round2.player)
    left join `round3` on admins.username = round3.player)
    left join `round4` on admins.username = round4.player)
    left join `round5` on admins.username = round5.player)
    WHERE admins.username <> 'admin' 
    GROUP BY admins.username

当我知道数据在我的表中时，mysql为空结果集

4 个答案: