Question

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, for each row that has a null value in Column B:

1. Take the value from column A
2. Check for the existence of that value in ANY row on column B
3. If there is a match, inject the value into column B (so that Column A and B have the same value)

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.

Answer 1

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)