带有CASE，GROUP BY和HAVING的SQL UPDATE

Question

下面的SELECT语句返回正确的数据。

SELECT stu.sc, stu.sn, COUNT(*) AS Total, 
    CASE 
     WHEN COUNT(*) = 3 Then 'Letter 1' 
     WHEN COUNT(*) = 4 Then 'Letter 2' 
     WHEN COUNT(*) = 5 Then 'Letter 3' 
     ELSE 'SARB' 
    END AS Letter 
FROM STU join att ON (stu.SC = att.SC and stu.SN = att.SN) 
WHERE att.al in ('c','t','u') 
GROUP by stu.sc, stu.sn 
HAVING COUNT(*) >= 3 

我需要执行基于该查询的更新，我似乎无法弄清楚。我在网上查看了多个与我需要的示例类似的示例，但我无法使其工作。我需要能够做这样的事情...

UPDATE stu 
SET stu.tru = 
    CASE 
    When COUNT(*) = 3 Then 'Letter 1' 
    When COUNT(*) = 4 Then 'Letter 2' 
    When COUNT(*) = 5 Then 'Letter 3' 
    ELSE 'SARB' 
    END 
FROM stu JOIN att 
    on (stu.sc = att.sc and stu.sn = att.sn) 
WHERE ATT.AL in ('c','t','u') 
GROUP BY stu.sc, stu.sn 
HAVING COUNT(*) >= 3 

我知道我不能使用GROUP BY和有直接的更新语句，但我看到他们用与集团选择的例子通过设置和加入之前。我无法让它工作。

感谢您的任何帮助。

Answer 1

WITH CTE AS (
    SELECT stu.sc, stu.sn, COUNT(*) AS Total, 
     CASE 
     WHEN COUNT(*) = 3 Then 'Letter 1' 
     WHEN COUNT(*) = 4 Then 'Letter 2' 
     WHEN COUNT(*) = 5 Then 'Letter 3' 
     ELSE 'SARB' 
     END AS Letter 
    FROM STU join att ON (stu.SC = att.SC and stu.SN = att.SN) 
    WHERE att.al in ('c','t','u') 
    GROUP by stu.sc, stu.sn 
    HAVING COUNT(*) >= 3 
) 
UPDATE stu 
SET stu.tru = cte.Letter 
FROM stu JOIN cte 
    on (stu.sc = cte.sc and stu.sn = cte.sn) 

Answer 2

UPDATE stu 
SET tru = sub.letter 
FROM 
    stu JOIN (
    SELECT stu.sc, stu.sn, 
     CASE 
      WHEN COUNT(*) = 3 THEN 'Letter 1' 
      WHEN COUNT(*) = 4 THEN 'Letter 2' 
      WHEN COUNT(*) = 5 THEN 'Letter 3' 
      ELSE 'SARB' 
     END AS Letter 
    FROM STU 
    JOIN att ON (stu.SC = att.SC AND stu.SN = att.SN) 
    WHERE att.al in ('c','t','u') 
    GROUP by stu.sc, stu.sn 
    HAVING COUNT(*) >= 3 
) sub ON stu.sc = sub.sc AND stu.sn = sub.sn