UNION

Combines the result of multiple queries.

Syntax

query_1 UNION [DISTINCT | ALL] query_2

Parameters

• DISTINCT (default): returns only unique rows. UNION is equivalent to UNION DISTINCT.
• ALL: combines all rows, including duplicates. Since de-duplication is memory intensive, queries using UNION ALL are faster and less memory-consuming. For better performance, use UNION ALL.

NOTE

Each query statement must return the same number of columns and the columns must have compatible data types.

Examples

Create tables select1 and select2.

CREATE TABLE select1(
    id          INT,
    price       INT
    )
DISTRIBUTED BY HASH(id);

INSERT INTO select1 VALUES
    (1,2),
    (1,2),
    (2,3),
    (5,6),
    (5,6);

CREATE TABLE select2(
    id          INT,
    price       INT
    )
DISTRIBUTED BY HASH(id);

INSERT INTO select2 VALUES
    (2,3),
    (3,4),
    (5,6),
    (7,8);

Example 1: Return all IDs in the two tables, including duplicates.

mysql> (select id from select1) union all (select id from select2) order by id;

+------+
| id   |
+------+
|    1 |
|    1 |
|    2 |
|    2 |
|    3 |
|    5 |
|    5 |
|    5 |
|    7 |
+------+
11 rows in set (0.02 sec)

Example 2: Return all the unique IDs in the two tables. The following two statements are equivalent.

mysql> (select id from select1) union (select id from select2) order by id;

+------+
| id   |
+------+
|    1 |
|    2 |
|    3 |
|    5 |
|    7 |
+------+
6 rows in set (0.01 sec)

mysql> (select id from select1) union distinct (select id from select2) order by id;
+------+
| id   |
+------+
|    1 |
|    2 |
|    3 |
|    5 |
|    7 |
+------+
5 rows in set (0.02 sec)

Example 3: Return the first three IDs among all the unique IDs in the two tables. The following two statements are equivalent.

mysql> (select id from select1) union distinct (select id from select2)
order by id
limit 3;
++------+
| id   |
+------+
|    1 |
|    2 |
|    3 |
+------+
4 rows in set (0.11 sec)

mysql> select * from (select id from select1 union distinct select id from select2) as t1
order by id
limit 3;
+------+
| id   |
+------+
|    1 |
|    2 |
|    3 |
+------+
3 rows in set (0.01 sec)