Syntax:
SELECT ...
UNION [ALL | DISTINCT] SELECT ...
[UNION [ALL | DISTINCT] SELECT ...]

UNION combines the result from multiple SELECT statements into a single
result set. The result set column names are taken from the column names
of the first SELECT statement.

Selected columns listed in corresponding positions of each SELECT
statement should have the same data type. (For example, the first
column selected by the first statement should have the same type as the
first column selected by the other statements.) If the data types of
corresponding SELECT columns do not match, the types and lengths of the
columns in the UNION result take into account the values retrieved by
all of the SELECT statements. For example, consider the following:

mysql> SELECT REPEAT('a',1) UNION SELECT REPEAT('b',10);
+---------------+
| REPEAT('a',1) |
+---------------+
| a             |
| bbbbbbbbbb    |
+---------------+

URL: https://dev.mysql.com/doc/refman/5.7/en/union.html

