为什么常规语句在DB2中有效，但我的准备语句却不起作用？

Question

I have a strange behavior when I try to use a following query with PreparedStatement

Query itself is following:

SELECT 
    CASE  
        WHEN TYPE LIKE '%Linux%' THEN 'Linux' 
        ELSE 'UNKNOWN'
    END AS OS, 
COUNT(*) AS TOTAL 

FROM COMPUTERS.OS 

GROUP BY 
    CASE   
        WHEN TYPE LIKE '%Linux%' THEN 'Linux'
        ELSE 'UNKNOWN' 
    END

And it works well. But when I create query like following:

SELECT 
    CASE  
        WHEN TYPE LIKE ? THEN ?
        ELSE 'UNKNOWN'

    END AS OS, 
COUNT(*) AS TOTAL 

FROM COMPUTERS.OS 

GROUP BY 
    CASE   
        WHEN TYPE LIKE ? THEN ? 
        ELSE 'UNKNOWN'
    END

And set it accordingly

String os = "Linux";
ps.setString(1, "%" +  os  + "%");
ps.setString(2, os);
ps.setString(3, "%" +  os  + "%");
ps.setString(4, os);

It doesn't work and throw a following error:

SQLCODE=-119, SQLSTATE=42803, SQLERRMC=TYPE A column reference in the SELECT or HAVING clause is invalid, because it is not a grouping column; or a column reference in the GROUP BY clause is invalid.

Somebody can me explain please what's wrong with my PreparedStatement?

Thank you.

Answer 1

This might have to do with parameters and how DB2 determines if the group by clause is identical to the columns in the select . A simple work around is to use a subquery:

SELECT OS, COUNT(*) AS TOTAL 
FROM (SELECT os.*,
             (CASE WHEN TYPE LIKE '%Linux%' THEN 'Linux' 
                   ELSE 'UNKNOWN'
              END) AS OS
      FROM COMPUTERS.OS os
     ) os
GROUP BY OS;

For the wildcard version, you would only put the wildcards in once and the compiler should accept the query.