Update column in set of records only if multiple rows exist with a given value in a different field? - sql

Question

My_Table would be something like this:
user_id shared_field bool_field
------- ------------ ----------
1 abc null
2 def null
3 ghi Y
4 ghi null
5 ghi null
6 abc Y
7 jkl null
If the bool_field changes for a user who shares the same shared_field with other users (such as user_id 3, 4, and 5 above), only that one user should have a 'Y'. The rest should have null values in the bool_field column. For example, if user_id 4 should now have the 'Y', I have to change user_id 4's bool_field to 'Y', and ensure that user_id 3 and 5 have bool_field values of null.
If the user doesn't share a shared_field value with anyone else, then that bool_field should be null (as in user_id 1 and 2 above).
Update: added a couple of lines to show that multiple user_ids could share a given shared_field (eg, 1 and 6 both have 'abc'; 3, 4, and 5 all have 'ghi' - only one 'abc' user should have a 'Y', and only one 'ghi' user should have a 'Y' and so on, while the rest have null in their bool_field column; user_ids that don't share a shared_field value, such as user_ids 2 and 7, should all have null in their bool_field column.) Clear as mud, right? ;)
This statement works:
UPDATE my_table
SET bool_field = (CASE
WHEN user_id = 4 THEN 'Y'
ELSE NULL
END)
WHERE shared_field = 'ghi'
AND (SELECT COUNT(shared_field)
FROM my_table
WHERE shared_field = 'ghi') > 1;
The question: is there some way that I can accomplish this same thing without knowing the shared_field value in advance? For example (and this doesn't work, of course) - Update: "of course" means I know this doesn't work because it is not correct Oracle syntax! The point is to give an idea of what I'm trying to do.
UPDATE my_table
SET bool_field = (CASE
WHEN user_id = 4 THEN 'Y'
ELSE NULL
END)
WHERE shared_field = (SELECT shared_field FROM my_table WHERE user_id = 4) as sharedVal
AND (SELECT COUNT(shared_field)
FROM my_table
WHERE shared_field = sharedVal) > 1;
Update: this is a regular SQL statement - I can't use a stored procedure.

Answer 1

First, rather than saying that something "doesn't work", it is generally helpful to tell us how it doesn't work. The query you posted, for example, appears to have syntax errors (as sharedVal is invalid because you can't assign an alias to an expression you're computing in the SELECT list). But it's not clear if "doesn't work" means that you're getting syntax errors (which we can relatively easily debug with the error message) or whether it means that the query runs but doesn't do what you want (which I would expect the query to do if the syntax errors were corrected) in which case knowing how the query isn't doing what you want would be helpful.
I would expect something like
UPDATE my_table a
SET bool_field = (CASE WHEN user_id = 4
THEN 'Y'
ELSE NULL
END)
WHERE shared_field = (SELECT shared_field
FROM my_table b
WHERE b.user_id = 4)
AND EXISTS( SELECT 1
FROM my_table c
WHERE a.shared_field = c.shared_field
AND a.user_id != c.user_id )
to work assuming that user_id is the primary key.

Answer 2

It's been awhile since I've worked on Oracle, so check to make sure that this query returns the list of records you want to update:
SELECT *
FROM my_table m
WHERE EXISTS (SELECT 1
FROM my_table
WHERE shared_field = m.shared_field
HAVING COUNT(shared_field) > 1);
If so, then this should work:
UPDATE my_table
SET bool_field = (CASE
WHEN user_id = 4 THEN 'Y'
ELSE NULL
END)
WHERE EXISTS (SELECT 1
FROM my_table
WHERE shared_field = m.shared_field
HAVING COUNT(shared_field) > 1);
You could also try this WHERE clause:
WHERE shared_field IN (SELECT shared_field
FROM my_table
GROUP BY shared_field
HAVING COUNT(shared_field) > 1);