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Bruce McMinn10,029 Points
UNION vs UNION ALL
I get different results for patron_id 3 in this query if I use UNION (COUNT of 4) or a UNION ALL (count of 8)
SELECT patron_id, COUNT(*) FROM loans_north WHERE returned_on IS NULL GROUP BY patron_id UNION--or UNION ALL SELECT patron_id, COUNT(*) FROM loans_south WHERE returned_on IS NULL GROUP BY patron_id ORDER BY patron_id;
But in this query I get basically the same count either way.
SELECT * FROM loans_south WHERE returned_on IS NULL AND patron_id = 3 UNION--or UNION ALL SELECT * FROM loans_north WHERE returned_on IS NULL AND patron_id = 3
It's late and I'm stumped. Any clues? Greg Kaleka, you out there?
Steven Parker224,836 Points
UNION works like it has a built-in
DISTINCT. It only returns unique rows based on the columns selected. But
UNION ALL returns all rows from both queries, unique or not.
Whether these return different result sets depends on the data being selected. It's entirely possible for the row count to be different when only a few columns are displayed, but the same when all columns are displayed.