sql查询在列的情况下连接多个表

Question

我有这样的SQL：

select case when.....end as colA, table2.col3
from table1
join table2 on table2.colB = colA

在if语句的情况下，还有一些其他的表列，这可行吗？我有错误说colA无效。

Answer 1

where或on子句中未理解列别名。

您可以使用子查询执行此操作，假设case仅涉及table1中的列：

select case when.....end as colA, t2.col3
from (select table1.*, (case when.....end) as colA
      from table1
     ) t1 join
     table2 t2
     on t2.colB = t1.colA;

否则，您可以将case语句放在on子句中：

select case when.....end as colA, t2.col3
from (select table1.*, (case when.....end) as colA
      from table1
     ) t1 join
     table2 t2
     on t2.colB = ( case when.....end );