第 30 题 (多选题)： Which two are true about a SQL statement using SET operators such as UNION.

A. The data type of each column returned by the second query must exactly match the data type of the corresponding column returned by the first query. B. The number, but not names, of columns must be identical for all SELECT statements in the query. C. The names and number of columns must be identical for all SELECT statements in the query. D. The data type of each column returned by the second query is automatically converted to the data type of the corresponding column returned by the first query. E. The data type group of each column returned by the second query must match the data type group of the corresponding column returned by the first query 显示答案