如何在Mysql Left Join中计数？

Question

我有表部门和员工。 架构是什么样子，

department(id, name) 
employee(id,depid,salary) 

我需要得到员工的数量在各个部门。 我在MySQL查询像

select d.depname,count(e.id) 
from employe as e left join department as d on d.depid 
where e.depid=d.depid group by d.depid; 

但结果集只包含具有雇员的部门。 我需要有0名员工的部门本身为零。

Answer 1

只是互换表名，

SELECT d.name, 
     COUNT(e.id) totalEmployeeCount 
FROM department AS d 
     LEFT JOIN employee AS e 
      ON e.depid = d.id 
GROUP BY d.id; 

为了进一步获得更多的知识有关加入，请访问以下链接：

• Visual Representation of SQL Joins

Answer 2

select d.depname,count(e.id) from employee as e 
right join department as d on d.depid = e.depid 
group by d.depid; 

至于原因，经过http://dev.mysql.com/doc/refman/5.0/en/join.html