What is the use of the Union of All?

Question

I do not ask the difference between them, my question is when do we need to use the “All Union”?

Answer 1

You would use UNION ALL when you really need several “copies” of strings that would otherwise be deleted when using UNION. It can also be faster on the query side, since the database engine does not need to determine what duplicates are between sets of results.

Answer 2

• UNION will remove duplicates
• UNION ALL does not remove duplicates

Example

SELECT 1 AS foo
UNION
SELECT 1 AS foo

= one row

SELECT 1 AS foo
UNION ALL
SELECT 1 AS foo

= two rows

Answer 3

:

mysql> select * from tmp1;
+------+
| a    |
+------+
| foo1 |
| foo2 |
+------+
2 rows in set (0.00 sec)

mysql> select * from tmp2;
+------+
| a    |
+------+
| foo2 |
| foo3 |
| foo4 |
+------+
3 rows in set (0.00 sec)

mysql> select * from tmp1 union select * from tmp2;
+------+
| a    |
+------+
| foo1 |
| foo2 |   # DUPLICATES REMOVED.
| foo3 |
| foo4 |
+------+
4 rows in set (0.00 sec)

mysql> select * from tmp1 union all select * from tmp2;
+------+
| a    |
+------+
| foo1 |
| foo2 |
| foo2 |    # DUPLICATES NOT REMOVED.
| foo3 |
| foo4 |
+------+
5 rows in set (0.00 sec)

UNION ALL?

, , , UNION ALL UNION.

Answer 4

, . UNION UNION ALL , UNION ALL .

Answer 5

Union all ,

Answer 6

Answer 7

UNION ALL () , ( ).

Answer 8

, , UNION , UNION. , UNION ALL , UNION ALL , , .

SELECT DISTINCT SELECT ALL, BTW.

Answer 9

UNION . UNION UNIONALL, .:)

Answer 10

Union Union all?

Ans: If you are looking for some data from two or more different tables (I mean, by relational relations), you can use it.