【多选题】
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 climinated by the UNION operator
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