Tip

A quick UNION ALL trick for DBAs

Let's say that you have two tables that have columns of the datatype TEXT...

master: tblmaincourse
child: tblcourselessonobjects
...and you need all DISTINCT records that are linked or not linked to child tables. What do you do? Using a left join with the distinct clause from master to child table will give an error: "The text, ntext, or image data type cannot be selected as DISTINCT."

To solve the problem, use "UNION ALL". I used this from a VB program to populate a list view. It shows all the courses that are linked/not linked to a lesson object.

select mc.*, 'Linked' as status from  tblmaincourse mc  where 
             mc.course_id  in ( select distinct cl.course_id from tblcourselessonobjects cl)
Union All select 
mc.*, 'Not Linked' as status from  tblmaincourse MC where 
             mc.course_id not in ( select distinct cl.course_id from tblcourselessonobjects cl)

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This was first published in February 2005

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