单选题

select * from tablename where columnA...

Question

select * from tablename where columnA=’a’ union select * from tablename where columnB=’b’ 与此查询语句等价的选项是

A.

select * from tablename where columnA=‘a‘ and columnB=‘b‘

B.

select * from tablename where columnA=‘a‘ or columnB=‘b‘

C.

select * from tablename where columnA=‘a‘

D.

select * from tablename where columnB=‘b‘