What should be considered when using UNION in MySQL?

When using UNION in MySQL, the following points need to be taken into consideration:

1. The two SELECT statements in UNION must have the same number of columns. If the columns do not match, you can use aliases or add virtual columns to make them match.
2. By default, UNION removes duplicate rows, but if you want to include duplicate rows, you can use UNION ALL.
3. The data types of the columns in the two SELECT statements of UNION must be compatible. If they are not compatible, you can use the CAST or CONVERT functions to perform type conversion.
4. UNION is only allowed to be used between SELECT statements and cannot be used with other statements such as INSERT, UPDATE, or DELETE.
5. The column names in the result set of a UNION are determined based on the column names in the first SELECT statement, and aliases can be used to set the column names in the result set.
6. The UNION will order the result set based on the order of the columns in the SELECT statement, but if a specific sorting rule is needed, the ORDER BY clause can be used.
7. You can use UNION multiple times to merge multiple SELECT statements.
8. The performance of UNION may not be as good as using other methods such as JOIN or subqueries.
9. Using UNION will sort and deduplicate the result set, so it may have a certain impact on performance. If sorting and deduplication are not necessary, consider using UNION ALL.

Overall, when using UNION, it is important to pay attention to issues such as the number of columns, data types of columns, handling duplicate rows, column names, and sorting of the result set. Additionally, it is important to consider the performance of UNION and use it in appropriate scenarios.