Difference Union and Union ALL In SQL | Part 4 SQL SERVER #sqlserver

Difference Union and Union ALL In SQL | SQL SERVER #sqlserver

In SQL, both UNION and UNION ALL are used to combine the results of multiple SELECT queries into a single result set. However, they have a key difference in how they handle duplicate rows:

UNION:
The UNION operator combines the results of the queries and removes any duplicate rows from the final result set.
If there are any duplicate rows between the queries, only one copy of the duplicate rows will be included in the result set.
Example:
Suppose we have two tables, "TableA" and "TableB," each with one column "Value." TableA has the values (1, 2, 3), and TableB has the values (2, 3, 4). Using UNION, the query below will give the result (1, 2, 3, 4), removing the duplicate value 2 and 3.

SELECT Value FROM TableA
UNION
SELECT Value FROM TableB;

UNION ALL:
The UNION ALL operator also combines the results of the queries, but it retains all rows from each query, including any duplicate rows.
If there are duplicate rows between the queries, all copies of the duplicate rows will be included in the result set.
Example:
Using UNION ALL with the same tables, the query below will give the result (1, 2, 3, 2, 3, 4), including all rows from both tables, including duplicates.

SELECT Value FROM TableA
UNION ALL
SELECT Value FROM TableB;
In summary, the main difference between UNION and UNION ALL is in how they handle duplicates. If you want to eliminate duplicate rows and get a distinct result set, use UNION. If you want to include all rows, even if there are duplicates, use UNION ALL. UNION ALL is generally faster because it doesn't need to check for duplicates, but the choice between them depends on the specific requirements of your query.

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