有没有办法让这个SQL更有效率？

Question

请考虑以下表格:

部门

deptid      (type:INT)
deptname    (type: TEXT)
hours       (type:INT)
active      (type:BIT)

雇员

empid       (type:INT)
empname     (type: TEXT)
deptid      (type: INT)
designation (type: TEXT)
salary      (type: INT)

编写查询以返回属于头数为4或更多的部门的员工的列名empname和deptname.记录应按empname的字母顺序返回

这是我的看法:

SELECT e1.empname, d.deptname from employee AS e1
FULL JOIN department AS d on e1.deptid = d.deptid
  WHERE e1.deptid IN(
    SELECT deptid FROM(
      SELECT e2.deptid, COUNT(e2.empid)
      FROM employee AS e2
      GROUP BY e2.deptid
      HAVING COUNT(e2.empid) >= 4
    )
  )
ORDER BY empname;

你会如何改进？

Answer 1

这个更短,也可能更快

SELECT e1.empname, d.deptname
from (
      SELECT e2.deptid
      FROM employee AS e2
      GROUP BY e2.deptid
      HAVING COUNT(e2.empid) >= 4
    ) G
inner join employee AS e1 on e1.deptid = G.deptid
INNER JOIN department AS d on d.deptid = G.deptid
ORDER BY e1.empname;

从分组开始.内部查询不需要COUNT.然后,加入两个表只是为了获取名称.

使用INNER JOIN是因为一旦计数完成,我们就已经知道了

1. 员工存在
2. 部门存在