SQL子查询作为具有多个where子句的列答案

【问题标题】：SQL subquery as column with multiple where caluseSQL子查询作为具有多个where子句的列
【发布时间】：2018-12-12 10:19:22
【问题描述】：

我有三张桌子School, Grade, Student。表Grade 有一个Foreign Key 列s_id 引用Primary Key 列id 的School 和表Student 有一个Foreign Key 列g_id 引用@987645332@ 列@987643331@ @ 表Grade。我需要选择school_name，身高低于170或高于180的学生人数。现在，我这样写选择查询。

SELECT School.name, School.id as sid, 
       (Select count(*) from Student inner join Grade on 
                     Student.g_id=Grade.id inner join School on 
                     Grade.s_id=School.id where School.id=sid and 
                     Student.height < 170) as under_170,  
       (Select count(*) from Student inner join Grade on 
                     Student.g_id=Grade.id inner join School on 
                     Grade.s_id=School.id where School.id=sid and Student.height > 180) as over_180
from School

我必须为每一列使用join。有更好的方法吗？

【问题讨论】：

标签： mysql sql join subquery

【解决方案1】：

以下使用 SUM() 聚合器，并且 MySQL 在对值求和以实现所需输出时将布尔值视为 1 或 0。

SELECT school.id, school.name, SUM(student.height < 170) AS under_170, SUM(student.height > 180) AS over_180
FROM school
INNER JOIN grade ON (school.id = grade.s_id)
INNER JOIN student ON (grade.id = student.g_id)
GROUP BY school.id

【讨论】：