SQL 查询 - 子查询返回多于一行 [英] SQL Query - Subquery returns more than one row

问题描述

表格:

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 子查询返回多于一行"错误.

I'm getting " MySQL Subquery Returns more than one row" error.

我知道此查询使用以下查询的解决方法:

I know a workaround for this query to use the following query:

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

然后使用 php 循环来处理结果并执行以下查询以获取 late_count 并将其回显:

Then using php loop to though the results and do below query to get the late_count and echo it out:

SELECT Count(laterecords.studentid) AS late_count FROM laterecords 

但我认为可能有更好的解决方案?

But i think there might be a better solution ?

推荐答案

简单的解决方法是在您的子查询中添加一个 WHERE 子句:

The simple fix is to add a WHERE clause in your subquery:

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)是使用连接:

A better option (in terms of performance) is to use a join:

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

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