Php 警告：子查询返回的行数超过1行

当我运行此查询时

SELECT a.sname,a.date,a.Roll_Number,b.branch,
                            COUNT(DISTINCT a.date) AS totaldays,

                            (SELECT COUNT(a.attendance_status)
                            FROM attendance as a , branch as b  
                            WHERE a.attendance_status='Present' and b.id='".$_GET['id']."' and  a.branch=b.branch  GROUP BY a.sid )
                             as present_days
                             FROM attendance as a , branch as b  
                             WHERE b.id='".$_GET['id']."'
                            and  a.branch=b.branch
                            GROUP BY a.sname ";

它显示了这个错误

警告：mysqli:：query:21000/1242:子查询返回的行数超过1行

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请帮助我更正此问题。提前感谢您

查看您的查询，您现在似乎不需要子查询，要避免此子查询，您应该避免出现错误

SELECT a.sname
  ,a.date,a.Roll_Number
  ,b.branch,
  COUNT(DISTINCT a.date) AS totaldays,
  COUNT(a.attendance_status) as present_days
  FROM attendance as a 
  INNER JOIN branch as b   ON a.attendance_status='Present' 
      and b.id='".$_GET['id']."' 
          and  a.branch=b.branch  
  GROUP BY a.sid 

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不管怎么说，你应该考虑到sqlinjection的风险。。您不应该在sql中使用php var，而应该使用prepared语句和binding参数

其中b.id='.$\u GET['id'].'因为您按分组，所以您的计数可以返回多行。这对于子查询是非法的，它们只能返回一行。我需要正确的查询。因为我是Mysql新手