SQL: Populating Column B where Column A has a match elsewhere in Column B


I’m somewhat of a newbie to SQL queries, especially anything containing logic, and although I've searched for hours finding the exact terms to search for is not easy in this case! I have a relatively simple one, I’m sure:

A table has 2 columns, and each row contains data about a function in a program. Some functions have a parent function associated (for grouping). Column A is the unique function ID. Column B indicates, when applicable, the parent function’s ID. All parent function IDs are independent and valid function IDs that exist elsewhere in column A.

For reporting purposes I need to list the functions grouped by their parent ID, listing the parent function with the child functions. I can easily report by parent function ID, but the problem is that a parent function does not know that it is a parent function because its column B is empty!

What I need to do is complete the value in Column B if it is empty and the function is referenced elsewhere as a parent function.

Otherwise stated, <strong>for each row</strong> that has a null value in Column B:

<ol><li>Take the value from column A</li> <li>Check for the existence of that value in ANY row on column B</li> <li>If there is a match, inject the value into column B (so that Column A and B have the same value)</li> </ol>

What I have: (Query: SELECT function_id, parent_function FROM functions)

FUNCTION_ID PARENT_FUNCTION 4 13 4 79 138 4 195 314 345 345

What I need to have:

FUNCTION_ID PARENT_FUNCTION 4 4 13 4 79 138 4 195 314 345 345 345

Any Ideas? I can't wait to get more familiar with SQL! Thanks ahead of time.


This should work for you:

UPDATE functions SET parent_function = function_id WHERE parent_function IS NULL AND function_id IN (SELECT parent_function FROM functions)

This will set parent_function equal to function_id where it has not yet been set, and where it appears somewhere in the parent_function column.

If you don't actually want to modify the table data but still return values that you need, you can use similar logic like this:

SELECT f.function_id, COALESCE(f.parent_function, f2.function_id) as parent_function FROM functions f LEFT JOIN functions f2 ON f.function_id = f2.function_id AND f2.function_id IN (SELECT parent_function FROM functions)


maybe you can compare the two table using EXCEPT or INTERSECT


more tutorials>:



How's this look?

select distinct t1.funx, t1.parent, case when t2.parent is null then t1.parent else t2.parent end as newparent from tbl t1 left outer join tbl t2 on t1.funx = t2.parent

<a href="http://sqlfiddle.com/#!3/55d23/15/1" rel="nofollow">sqlFiddle</a>


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