php - 当我知道数据在我的表中时，mysql结果集为空

Question

我有 5 个表正在运行 SELECT 语句，当所有表中都有数据时，一切正常，但如果只有 1 个表中有数据，我会得到零结果吗？当并非所有表都填充了数据时，如何使查询正常工作。现在正让我头疼，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？您需要使用外连接来代替:

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

我不确定您是否真的需要group by，一般来说它与聚合一起使用(或者有时用于获取不同记录)。