Combining two result sets can remove visible repetitions even when each source contains legitimate separate events. In Access, the choice between UNION and UNION ALL changes that result. Decide whether the output is a set of different displayed values or a list retaining every contributing row.

Use a disposable database with two small tables. Let EastEvents contain Item A with Units 2 and Item B with Units 1. Let WestEvents contain Item A with Units 2 and Item C with Units 3. The two A records may be separate events whose selected fields happen to look identical.

Compare the two query meanings

Microsoft documents that UNION suppresses duplicate result records, while UNION ALL retains all records. Each SELECT must request the same number of fields. Use an explicit field list in a copied SQL-view query so the correspondence is visible:

SELECT Item, Units FROM EastEvents
UNION ALL
SELECT Item, Units FROM WestEvents;

For this sample, retaining all rows means four records and a quantity total of eight. With UNION in place of UNION ALL, the repeated selected pair A and 2 appears once: three rows remain, totalling six. The lower total follows from the result rule, not necessarily from a missing source table.

Decide what must distinguish a record

If the report describes individual events, include the fields needed to identify those events and preserve their origin when the source identifiers are only locally unique. If the report deliberately asks for different Item-and-Units combinations, duplicate suppression may match its purpose.

Do not treat UNION as a way to repair conflicting source records. It compares the selected results; omitting an event identifier can make different events appear identical. Conversely, adding a field solely to force more rows does not establish a sound identity rule.

Run both alternatives on the small copy and inspect the repeated pair explicitly. Then reconcile the real result against its source queries, documenting whether repetitions are expected. Keep the source records intact while making this decision: a SELECT-based union changes what the query displays, not the underlying events.

Sources: Microsoft: Access UNION operation.