DB2中的列别名的内部联接

Question

I have a more complex query with inner join that throws the error:

[Error Code: -206, SQL State: 42703] DB2 SQL Error: SQLCODE=-206, SQLSTATE=42703, SQLERRMC=TT.ALIAS, DRIVER=4.13.127. 2) [Error Code: -727, SQL State: 56098] DB2 SQL Error: SQLCODE=-727, SQLSTATE=56098, SQLERRMC=2;-206;42703;TT.ALIAS, DRIVER=4.13.127

the simple version of the query that doesn't work is:

select column1 as alias from table1 tt
inner join table1 ts
on tt.alias = ts.column1

this simple query doesn't make sense but is a simple version of the query I'm trying to run. This was supposed to run with no errors right? If I remove the alias it runs with no problem..

Thanks in advance

Answer 1

You cannot use a column alias in an on or where clause. You can do:

select column1 as alias
from table1 tt inner join
     table1 ts
     on tt.column1 = ts.column1;

You could also do:

select alias
from (select column1 as alias
      from table1
     ) tt inner join
     table1 ts
     on tt.alias = ts.column1;

During the compile phase, the queries are evaluated by clause, with the from clause first, then then where and select . In other words, the compiler doesn't "know" what is in the select when it evaluates the from .