A quick UNION ALL trick for DBAs

Use UNION ALL to get all the records from parent and child tables that have columns of text data type.

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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