SQL查询-子查询返回多个行

SQL查询-子查询返回多个行

简单的解决方法是

WHERe

在子查询中添加一个子句：

SELECt    studentid,    latetime,    reason,    (SELECT COUNT(*)     FROM laterecords AS B     WHERe A.studentid = B.student.id) AS late_count FROM laterecords AS A

一个更好的选择（就性能而言）是使用联接：

SELECt    A.studentid,    A.latetime,    A.reason,    B.totalFROM laterecords AS AJOIN (    SELECt studentid, COUNT(*) AS total    FROM laterecords     GROUP BY studentid) AS BON A.studentid = B.studentid