Question

当我写下面的查询时

SELECT Op.MRNO,
       M.PatientName,
       (
           SELECT COUNT(MRNO) AS Visit
           FROM HMS_Reg_OPRegister
           GROUP BY MRNO
       )
FROM HMS_Reg_Patient M,
     HMS_Reg_OPRegister Op
WHERE Op.MRNO = M.MRNO

我收到了这个错误：

Msg 512，Level 16，State 1，Line 14 Subquery返回的值超过1 值。当子查询遵循=，！=，＆lt;，＆lt; =，＆gt; =或子查询用作表达式时，不允许这样做。

Answer 1

SELECT Op.MRNO,
       M.PatientName,
       Op.Visit
FROM dbo.HMS_Reg_Patient M
JOIN (
    SELECT MRNO, COUNT(MRNO) AS Visit
    FROM dbo.HMS_Reg_OPRegister
    GROUP BY MRNO
) Op ON Op.MRNO = M.MRNO

Answer 2

看起来好像你想要：

SELECT M.MRNO,
       MAX(M.PatientName) AS PatientName,
       COUNT(*) AS Visit
FROM HMS_Reg_Patient M
JOIN HMS_Reg_OPRegister Op ON Op.MRNO = M.MRNO
GROUP BY M.MRNO

当您已经拥有在主查询中计算的表时，没有必要使用子查询来计算组中值的数量。

如何在连接查询中编写子查询？

2 个答案: