The character set of the expression in the first SELECT statement determines the character set of the entire query. In the following example, the character set of char_expression_1 is used as the character set for the entire query:
SELECT char_expression_1 FROM table_1 UNION SELECT char_expression_2 FROM table_2;
If char_expression_2 contains characters not included in the character set of char_expression_1, an error can result.
For example, if char_expression_1 is CHARACTER SET LATIN but char_expression_2 is CHARACTER SET UNICODE, trying to translate char_expression_2 from UNICODE to LATIN causes an error if char_expression_2 has multibyte characters.