結合クエリ内でサブクエリを書く方法は？

Question

私は以下のクエリ

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 

を書くとき、私はこのエラーを得た：あなたがしたいかのように見えます

Msg 512, Level 16, State 1, Line 14 Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <=, >= or when the subquery is used as an expression.

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

を：

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 

はそれを使用する必要はありませんサブクエリを使用して、グループ内の値の数を取得します。すでにmを数えているテーブルがある場合クエリ。