Syntax:
query_expression_body UNION [ALL | DISTINCT] query_block
    [UNION [ALL | DISTINCT] query_expression_body]
    [...]

query_expression_body:
    See 

UNION combines the result from multiple query blocks into a single
result set. This example uses SELECT statements:

mysql> SELECT 1, 2;
+---+---+
| 1 | 2 |
+---+---+
| 1 | 2 |
+---+---+
mysql> SELECT 'a', 'b';
+---+---+
| a | b |
+---+---+
| a | b |
+---+---+
mysql> SELECT 1, 2 UNION SELECT 'a', 'b';
+---+---+
| 1 | 2 |
+---+---+
| 1 | 2 |
| a | b |
+---+---+

URL: https://dev.mysql.com/doc/refman/8.4/en/union.html

