Question

出于某种原因，我无法通过查询进行CTE：

SELECT LetterGrade, ES.Classification 
FROM Enrollment AS EN 
JOIN Students AS ES ON EN.StudentID = ES.StudentID

本声明：

WITH [DERIVED] AS
(
    SELECT LetterGrade, ES.Classification 
    FROM Enrollment AS EN 
    JOIN Students AS ES ON EN.StudentID = ES.StudentID
)

引发错误：

Msg 102，Level 15，State 1，Line 5
'）'
附近的语法不正确

有什么想法吗？

Answer 1

有两种方法。

选项1：使用内部查询：

select ...
from (SELECT LetterGrade, ES.Classification
      FROM Enrollment AS EN
      JOIN Students AS ES ON EN.StudentID = ES.StudentI) as Derived
where ...

此语法非常便于携带。它适用于所有主要的SQL数据库。

选项2：使用较少支持的CTE语法：

with Derived as (
    SELECT LetterGrade, ES.Classification
    FROM Enrollment AS EN
    JOIN Students AS ES ON EN.StudentID = ES.StudentI)
select ...
from Derived
where ...

这样便携性较差（例如postgres支持它，mysql不支持）它也可以更容易阅读，如果多次提到CTE可以提供编码和性能优势。

无法使用join创建公用表表达式

1 个答案: