mysql - 嵌套 MySQL 查询失败，错误代码为 `Error Code 1060 Duplicate column name ' xxx'`

Question

所以我有三个表:tabe_1、table_2、table_3，我将使用字段 A 来映射前两个表，因为它包含在table_1 和 table_2，然后将 table_3 与字段 B 和 C 连接，然后添加一些过滤器(例如: where 语句)，查询是:

SELECT *
FROM 
(select *
from table_1 t1 
join table_2 t2 
on t1.A = t2.A
join table_3 t3
on t1.B  = t3.B 
and t1.C = t3.C) AS output_table

WHERE output_table.xx = xxx

这给了我错误:错误代码:1060。重复的列名称“A”

但是如果我只查询子查询:

select *
from table_1 t1 
join table_2 t2 
on t1.A = t2.A
join table_3 t3
on t1.B  = t3.B 
and t1.C = t3.C

这将返回output_table，有人可以看看嵌套查询发生了什么吗？谢谢。

Answer 1

因为您的 SQL 查询需要能够区分子查询字段，以便将其视为表类型记录源。

以下是所发生情况的示例:

with table_1 as (select 0 A, 0 B, 0 C),
     table_2 as (select 0 A, 0 D),
     table_3 as (select 1 A, 0 B, 0 C)
SELECT *
FROM 
(select *
from table_1 t1 
join table_2 t2 
on t1.A = t2.A
join table_3 t3
on t1.B  = t3.B 
and t1.C = t3.C) AS output_table
WHERE output_table.D = 0;

此操作失败，因为子查询具有字段 t1.A/t1.B/t1.C 和 t2.A/t2.D 和 t3.A/t3。 B/t3.C。

如果不将其设为子查询，那么MySQL引擎不需要区分字段，并且可以不区分地输出包含所有字段的记录。 根据您的情况，有效的查询:

with table_1 as (select 0 A, 0 B, 0 C),
     table_2 as (select 0 A, 0 D),
     table_3 as (select 1 A, 0 B, 0 C)
select *
from table_1 t1 
join table_2 t2 
on t1.A = t2.A
join table_3 t3
on t1.B  = t3.B 
and t1.C = t3.C;

因此，为了避免出现此问题，请从子查询中精确选择所需的字段，如下所示:

with table_1 as (select 0 A, 0 B, 0 C),
     table_2 as (select 0 A, 0 D),
     table_3 as (select 1 A, 0 B, 0 C)
SELECT *
FROM 
(select t1.*, t2.D
from table_1 t1 
join table_2 t2 
on t1.A = t2.A
join table_3 t3
on t1.B  = t3.B 
and t1.C = t3.C) AS output_table
WHERE output_table.D = 0;

更清楚地说，假设您想要将另一个表与子查询连接起来((subquery) AS output_table join another_table t4 on t4.A = output_table.A，MySQL 引擎如何确定它应该使用 output_table 中的哪一个字段 A 与 t1.A (0) 和 T3.A (1) 之间的 another_table 连接？它不能，除非您在子查询中仅指定一个字段“A”。