SELECT / UNION Giving Different Results

We've had a very strange (and possibly troubling) event happen.

There is a table created and loaded by a SELECT statement which UNIONs about 20 tables together. While an analyst was analyzing the data she discovered there were rows in the source tables that never made it into the table created by the UNIONS and hence never loaded into the production table.

When I was testing I reran the SELECT statement by copying it from the script it was in into SQL*PLUS and created a test table using the same source tables the original SELECT did. When I checked to see how many were in the test table but not in the production table I discovered 4,865 rows.

Has anybody ever experienced anything like this? Using the same source tables the SELECT / UNION will one time miss almost 5,000 rows that it picks up the next time. We're at a loss trying to figure this out so any help/advice would be greatly appreciated.

I know examples may be helpful but the information in the tables is all Protected Health Information so I can't provide any. Received on Fri Jun 29 2007 - 20:18:01 CDT