mysql - 左连接出现 "unknown column"错误

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SELECT *
FROM `placement_ans` A, `placement_quiz` B
left join `placement_time` C on (A.state=C.state)
where
A.no_place=C.no_place and A.no_stu=C.no_stu and A.no_place=B.no_place and A.place_q=B.no_q and YEAR(A.place_date) = YEAR(curdate()) and MONTH(A.place_date) = MONTH(curdate()) and A.no_place=208 and A.no_stu=793 order by A.place_q ASC

错误是“on 子句”中的未知列“A.state”

我也尝试过使用

SELECT *
FROM `placement_ans` A, `placement_quiz` B
left join `placement_time` C on (placement_ans.state=C.state)
where
A.no_place=C.no_place and A.no_stu=C.no_stu and A.no_place=B.no_place and A.place_q=B.no_q and YEAR(A.place_date) = YEAR(curdate()) and MONTH(A.place_date) = MONTH(curdate()) and A.no_place=208 and A.no_stu=793 order by A.place_q ASC

但它似乎不想工作

我有一张像这样的 table

placement_time
state   num
100     10

和

placement_ans
state   something
100     11          
100     22          
55      33          
22      55          
33      77          

我想打印出一个表格

placement_ans
state   something    num
100     11           10
100     22           10
55      33           null
22      55           null
33      77           null

Answer 1

根据给定的示例数据集和预期结果，您不需要引用表placement_quiz，可以这样做

select a.*,b.num from placement_ans a
left join placement_time b  on b.state = a.state