DB2子句在where子句中查询别名

Question

Is there a way where we could query using aliases

SELECT 
   'X' AS AVC
 FROM 
   sysibm.sysdummy1
 WHERE AVC= 'X'

I am just looking for a way , I could query for alias column.

I am getting the below error.

Error: DB2 SQL Error: SQLCODE=-206, SQLSTATE=42703, SQLERRMC=AVC, 
DRIVER=3.63.75
SQLState:  42703
ErrorCode: -206
Error: DB2 SQL Error: SQLCODE=-514, SQLSTATE=26501, SQLERRMC=SQL_CURLH200C1, 
DRIVER=3.63.75
SQLState:  26501
ErrorCode: -514

Any input would be helpful !

Thanks !

Answer 1

you can't use alias in where condition because the sql engine eval the query clause (FROM, WHERE, SELECT...) in a specific order and the select clause in evaluated after the where clause

so at the moment of the where evalution the column alias in not know by th db engine

You must repeat the code

SELECT 'X' AS AVC
FROM  sysibm.sysdummy1
WHERE X' = 'X' 

Answer 2

You can't. This is a general rule of SQL. In practice, you can think of the reason being that the where is parsed before the select , so the aliases are not known.

The general solution is to use a subquery or CTE:

SELECT t.*
FROM (SELECT 'X' AS AVC
      FROM sysibm.sysdummy1
     ) t
WHERE AVC = 'X';

My preferred solution is to use a lateral join, but I don't think DB2 supports either APPLY or LATERAL .

Answer 3

The engine does not know of table or column aliases until it is finished gathering the data. You can use them in ORDER BY statement within the same query but that's it. You can however, use a Common Table Expression also known as a CTE :

WITH myCTE AS (
SELECT 'X' AS AVC
 FROM sysibm.sysdummy1
)
SELECT *
  FROM myCTE
    WHERE AVC= 'X'

This is somewhat of a long way to do it but unfortunately that is it.