Scope: managed applications, mobile applications, and ordinary applications.
In most cases, when you need to combine the results of two or more queries into a single result set, employ UNITE ALL instead of UNITE. The recommendation is based on the algorithm of the UNITE clause, which searches for and removes duplicates from the united result even when duplicates are impossible by the query design.
Employ UNITE only when removing duplicates from the result is required.
Correct:
SELECT
GoodsReceipt.Ref
FROM
Document.GoodsReceipt AS GoodsReceipt
UNION ALL
SELECT
GoodsSale.Ref
FROM
Document.GoodsSale AS GoodsSale
Incorrect:
SELECT
GoodsReceipt.Ref
FROM
Document.GoodsReceipt AS GoodsReceipt
UNION
SELECT
GoodsSale.Ref
FROM
Document.GoodsSale AS GoodsSale