What is the difference between UNION and UNION ALL?

Question

What is the difference between UNION and UNION ALL?

Answer 1

(From Microsoft SQL Server Book Online)

UNION [ALL]

Specifies that multiple result sets are to be combined and returned as a single result set.

ALL

Incorporates all rows into the results. This includes duplicates. If not specified, duplicate rows are removed.

UNION will take too long as a duplicate rows finding like DISTINCT is applied on the results.

SELECT * FROM Table1
UNION
SELECT * FROM Table2

is equivalent of:

SELECT DISTINCT * FROM (
    SELECT * FROM Table1
    UNION ALL
    SELECT * FROM Table2) DT

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A side effect of applying DISTINCT over results is a sorting operation on results.

UNION ALL results will be shown as arbitrary order on results But UNION results will be shown as ORDER BY 1, 2, 3, ..., n (n = column number of Tables) applied on results. You can see this side effect when you don't have any duplicate row.