SQL 查询 - 子查询返回多行 - SQL Query - Subquery returns more than one row

SQL Query - Subquery returns more than one row

本文关键字：查询返回 SQL | 更新日期: 2023-09-27

Table：

laterecords
-----------
studentid - varchar
latetime - datetime
reason - varchar

我的查询：

SELECT laterecords.studentid,
laterecords.latetime,
laterecords.reason,
( SELECT Count(laterecords.studentid) FROM laterecords 
      GROUP BY laterecords.studentid ) AS late_count 
FROM laterecords

我收到" MySQL子查询返回多行"错误。

我知道此查询使用以下查询的解决方法：

SELECT laterecords.studentid,
laterecords.latetime,
laterecords.reason 
FROM laterecords

然后使用 php 循环遍历结果并执行以下查询以获取late_count并将其回显出来：

SELECT Count(laterecords.studentid) AS late_count FROM laterecords

但我认为可能有更好的解决方案？

简单的解决方法是在子查询中添加一个WHERE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

更好的选择（就性能而言）是使用联接：

SELECT
    A.studentid,
    A.latetime,
    A.reason,
    B.total
FROM laterecords AS A
JOIN 
(
    SELECT studentid, COUNT(*) AS total
    FROM laterecords 
    GROUP BY studentid
) AS B
ON A.studentid = B.studentid