152.Which statement is true regarding the UNION operator?
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152.Which statement is true regarding the UNION operator?
A.By default, the output is not sorted.
B.NULL values are not ignored during duplicate checking.
C.Names of all columns must be identical across all SELECT statements.
D.The number of columns selected in all SELECT statements need not be the same.
答案：B
解析：union就是合并，排除重复数据，不会忽略null，会排序(由于会滤重，所以会排序），合并的查询列数需要相等，类型必须相等或者隐式转换,
不支持long类型
A.By default, the output is not sorted.
B.NULL values are not ignored during duplicate checking.
C.Names of all columns must be identical across all SELECT statements.
D.The number of columns selected in all SELECT statements need not be the same.
答案：B
解析：union就是合并，排除重复数据，不会忽略null，会排序(由于会滤重，所以会排序），合并的查询列数需要相等，类型必须相等或者隐式转换,
不支持long类型
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