第 3 题 (多选题)： Which three statements are true regarding the UNION and UNION ALL operators.

A. The number of columns selected by the first SELECT statement can be greater than the number selected in subsequent SELECT statements. B. Duplicates are eliminated automatically by the UNION ALL operator. C. The number of columns selected in each SELECT statement must be identical . D. NULLS are not ignored during duplicate checking. E. The names of columns selected in each SELECT statement must be identical. F. The names of columns selected in each SELECT statement can be identical . G. Duplicates can optionally be eliminated by the UNION operator 显示答案