每日一练013

每日一练013

如下哪两个说法正确：A.The expressions in the SELECT lists must match in number.B.Parentheses may not be used to alter the sequence of execution.C.The data type of each column in the second query must match the data type of its corresponding column in the first query.D.The ORDER BY clause can be used only once in a compound query, unless a UNION ALL operator is used答案：AC关于sql的知识点需要记忆，实践和理解A.集合运算中，select列表需要在列的数量上一致（感觉选项中表述不完整）B.括号可能不会改变执行顺序（就类比 1+(2X3)和1+2X3，并没有改变执行顺序）C.查询列类型必须一致或可以转换D.order by在sql中只能使用一次这道题放在这个章节里应该选AC吧，但是就题而言个人还是偏向于BC