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/*[例53]  查询学生姓名与教师姓名的差集
    对集合操作结果的排序
    [例53] 错误写法
     SELECT   *
        FROM    Student
        WHERE Sdept= 'CS'
        ORDER BY Sno
        UNION
        SELECT *
        FROM    Student
        WHERE Sage<=19
        ORDER BY  Sno；
正确写法
        SELECT *
        FROM Student
        WHERE Sdept= 'CS'
        UNION
        SELECT *
        FROM Student
        WHERE Sage<=19
        ORDER BY 1；


*/
     SELECT *
        FROM Student
        WHERE Sdept= 'CS'
        UNION
        SELECT *
        FROM Student
        WHERE Sage<=19
        ORDER BY 2