DB2 - 带有 DISTINCT 子句的 LISTAGG() - 不起作用？

Question

My query has a column with a small number of values in it, and I need to display them in a single field for each grouped result set - eg if I had an employee in 3 different departments, I'd want to see something like

EMPID          DEPTS                                   SOMENUMBER  SOMEOTHERNUMBER
------         ------                                  ----------- ---------------
0001           ACCOUNTING, CUST SERVICE, CALL CENTER        100           200

The problem is when there are multiple duplicate departments for the employee. I see numerous questions on how to figure this out for Oracle and other DMBSs, but nothing specific to DB2. IBM's own documentation at https://www.ibm.com/support/knowledgecenter/SSFMBX/com.ibm.swg.im.dashdb.sql.ref.doc/doc/r0058709.html says: - "If DISTINCT is specified, duplicate string-expression values are eliminated.", and - "If DISTINCT is specified for LISTAGG, the sort-key of the ORDER BY specification must match string-expression (SQLSTATE 42822). If string-expression is implicitly cast, the sort-key must explicitly include a corresponding matching cast specification.".

As a simplistic example:

with mylist (field1) as 
    ( values 'A','A','B','C','D','A','C'
    )
    select listagg (distinct field1, ', ') 
        within group (order by field1) 
        from mylist;

The info on the IBM page suggests that this should return 'A, B, C, D' but instead I get 'AAA B, C. C, D'.

I should be able to get around this by having a subquery do some initial rolling up - eg

SELECT EMPLOYEE, LISTAGG(DEPARTMENT,', '),  
    SUM(SOMENUMBER) SOMENUMBER, SUM(SOMEOTHERNUMBER) SOMEOTHERNUMBER
from (
    SELECT EMPLOYEE, DEPARTMENT, SUM(SOMENUMBER) SOMENUMBER, SUM(SOMEOTHERNUMBER) SOMEOTHERNUMBER
    FROM EMPLOYEES GROUP BY EMPLOYEE, DEPARTMENT)
) GROUP BY EMPLOYEE

and in fact I guess that's what I'll do, but the IBM documentation sure suggests the DISTINCT ought to do the trick. What am I missing?

Answer 1

You haven't specified, but if you use DB2 9.7 LUW, like me, LISTAGG(DISTINCT .. doesn't (yet) work. You have to workaround it by XML functions, eg:

with mylist (field1) as 
    ( values 'A','A','B','C','D','A','C'
    )
    select XMLCAST(
             XMLQUERY('string-join(distinct-values($x//row), ", ")'
                      PASSING XMLGROUP(field1 ORDER BY field1) AS "x"
           ) AS VARCHAR(200))
        from mylist