UNION and UNION ALL is the way to select records from different tables having the same number and type of columns. Union internally use DISTINCT to remove the duplicate records while Union All don't use distinct and shows all the records even duplicate records.
-- Table A -- Table B -- Table C
ID VALUE ID VALUE ID VALUE
1 A 1 A 1 A
2 B 2 B 2 B
3 C 3 P 3 X
We will use above three tables A, B and C to understand union and union all.
Lets do the UNION with tables A and B to understand the UNION
SELECT ID, VALUE FROM A
UNION
SELECT ID, VALUE FROM B
As I said UNION internally use DISTINCT so result will have all the unique records, here is the output
ID VALUE
1 A
2 B
3 C
3 P
Now we will see the UNION ALL with table A and B again
SELECT ID, VALUE FROM A
UNION ALL
SELECT ID, VALUE FROM B
As we know union all shows all the records, and never used distinct so here is the result
ID VALUE
1 A
2 B
3 C
1 A
2 B
3 P
Let's see some unseen results, say you are doing UNION ALL between A and B and then UNION with table C like this
SELECT ID, VALUE FROM A
UNION ALL
SELECT ID, VALUE FROM B
UNION
SELECT ID, VALUE FROM C
So In this case A and B will select all the records and when it will do the UNION with C then all the records will be distinct, it is same as A union B union C, here is the result
ID VALUE
---- ------
1 A
2 B
3 C
3 P
3 X
What would be the result if we will use A union B union all C?, A union B will give distinct records and then union all with table C, see the result
ID VALUE
---- -------
1 A
2 B
3 C
3 P
1 A
2 B
3 X
Performance, which will perform better? Union All will always perform better than Union, when you're not concerned about eliminating duplicate records because you're avoiding an expensive distinct sort operation
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Having 13+ years of experience in Microsoft Technologies (C#, ASP.Net, MVC and SQL Server). Worked with Metaoption LLC, for more than 9 years and still with the same company. Always ready to learn new technologies and tricks.
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| By Ali Adravi On 14 Jun, 13 Viewed: 584 |
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