SQL-2条带有UNION和WITH的SELECT语句

Question

I have a WITH statement (I want to do some calculations on the results that why I am using with) and a SELECT .

Let's say that the SELECT statement is collecting the 20 students with the best grade.

Now I want to get the 20 students with the worst grade so I tried another

WITH cteGrades AS 
( 
    SELECT *
    FROM tbl_students
)
--Selecting best Grade 
SELECT TOP(20)
    cte.name, cte.surname, cte.grade
FROM 
    cteGrade cte
ORDER BY 
    cte.grade DESC

UNION

--Selecting worst Grade 
SELECT TOP(20)
    cte.name, cte.surname, cte.grade
FROM 
    cteGrades cte
ORDER BY 
    cte.grade ASC

I get the error:

Incorrect syntax near the keyword 'UNION'.

Answer 1

You should remove order by in the union clause. Do the ordering after the union on the entire result-set. Or have the queries as cte s and perform a union on the results.

 WITH cteGrade AS 
          ( 
            SELECT  *
            FROM tbl_students
           )
            --Selecting best Grade 
 , top20 as 
            (SELECT TOP(20)
                 cte.name
                ,cte.surname
                ,cte.grade
            FROM cteGrade cte
            ORDER BY cte.grade DESC)
 , last20 as 
             (
            --Selecting worst Grade 
            SELECT TOP(20)
                 cte.name
                ,cte.surname
                ,cte.grade
            FROM cteGrade cte
            ORDER BY cte.grade ASC
             )
 select name, surname, grade from top20
 union all
 select name, surname, grade from last20

Answer 2

WITH cteGrades AS 
( 
SELECT  *
      ,ROW_NUMBER() OVER (ORDER BY grade DESC) Best_Grades
      ,ROW_NUMBER() OVER (ORDER BY grade ASC ) Worst_Grades
FROM tbl_students
)
SELECT   cte.name
        ,cte.surname
        ,cte.grade
FROM cteGrades cte
WHERE Best_Grades  <= 20
  OR  Worst_Grades <= 20
ORDER BY cte.grade DESC

Answer 3

Your original query is just missing parentheses around the subqueries:

WITH cteGrades AS ( 
      SELECT *
      FROM tbl_students
     )
--Selecting best Grade 
(SELECT TOP(20) cte.name, cte.surname, cte.grade
 FROM cteGrade cte
 ORDER BY cte.grade DESC
) UNION
(SELECT TOP(20) cte.name, cte.surname, cte.grade
 FROM cteGrades cte
 ORDER BY cte.grade ASC
);

Rewriting to use CTEs or row numbers is also possible, but there is really no reason to do that.

Actually, you don't need a CTE at all for this:

(SELECT TOP(20) s.name, s.surname, s.grade
 FROM tbl_students s
 ORDER BY s.grade DESC
) UNION
(SELECT TOP(20) s.name, s.surname, s.grade
 FROM tbl_students s
 ORDER BY s.grade ASC
);