这一条sql哪里错了呀？

SELECT a.*,b.custom_name FROM zqwl_receipt a , zqwl_custom b LEFT JOIN b WHERE a.id= b.id

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from a left join b on a.id=b.id

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SELECT zqwl_receipt.*,zqwl_custom.custom_name from xxx LEFT JOIN xxx

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你的FROM 去哪里了

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没有 from 表
没有 on 来筛选这两个表的关系。

SELECT zqwl_receipt.*,zqwl_custom.custom_name from zqwl_receipt LEFT JOIN zqwl_custom on zqwl_custom.字段= zqwl_receipt.字段 WHERE zqwl_receipt.id= zqwl_custom.id

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基本语法错误

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left join的用法一般是先查询主表再join其他表的，比如

select a.*, b.custom_name from zqwl_receipt a
left join zqwl_custom b on a.id = b.id

如果你希望a、b两个表结合查询的结果再做关联查找的话，或许可以用子查询的方式

select * from (
  select a.*, b.custom_name from zqwl_receipt a, zqwl_custom b where a.id = b.id
) t
left join zqwl_custom t2 on t2.id = t.id

但是如果写出这样的语句来，最好要要思考a, b两个表的设计问题，
或者这完全就是没有必要的