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Asked: 2026-05-23T23:01:12+00:00

Currently I have UNION’s between 3 different select statements giving me three rows. What

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Currently I have UNION’s between 3 different select statements giving me three rows. What I’m needing is to modify this to have the union inside the from clause so that I can generate more columns (if I understand the functionality).

Basically what is going on is that the existing framework is designed and built to have a single row of data returned to it and will require heavy modification to handle a multi-row result set (everything is getting parsed to xml before being passed to the front-end).

My biggest issue (I believe) is being able to differentiate between the three sub selects inside the primary.

Guarantees in the select

1) Each select inside the from will only produce a single row result set.

2) All result sets from inside of the from will have same column count and column names (inherent of union I believe)

For example…
I currently have

SELECT * FROM A
    UNION
SELECT * FROM B
    UNION
SELECT * FROM C

Doing it this way produces a three row result set.

What I’m wanting if possible is….

SELECT cost as CurrentSelectedCost, /* from first select */
       cost as PreviousCost, /* from second select */
       cost as NextCost /* from third select */
FROM (
    SELECT * FROM A
        UNION
    SELECT * FROM B
        UNION
    SELECT * FROM C
)

Now I’m guessing that I will need to alias the different select statements that are within the from clause, but I’m having issues getting that to function. The examples that I’ve found on here didn’t seem to address the need to have all select statements inside of from differentiated. If this has been answered on here a link will suffice no reason to re-invent the wheel (I may just not know the terminology to search for) Also the database is a DB2 instance running on an iSeries

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1 Answer

  1. Editorial Team

    You need to unify the columns retrieved from the three SELECT statements.

    SELECT CurrentSelectedCost,
       PreviousCost, 
       NextCost 
FROM (
    SELECT cost as CurrentSelectedCost, 0 as PreviousCost, 0 as NextCost FROM A
        UNION
    SELECT 0 as CurrentSelectedCost, cost as PreviousCost, 0 as NextCost  FROM B
        UNION
    SELECT 0 as CurrentSelectedCost, 0 as PreviousCost, cost as NextCost  FROM C
) as COSTS
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