部

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这些列的员工的empname和deptname列人头数为4或以上的部门。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，也可能表现得更快

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。

INNER JOIN 被使用，因为一旦计数完成，我们就已经知道了

员工存在
部门存在

Answer 2

尝试此查询，它将正常工作。

select empname,deptname from employee,department
where 
employee.deptid=department.deptid and employee.deptid
in
(
  select deptId from employee group by deptid having count(*)>=4
) 
order by empname

有没有办法让这个SQL更有效率？

部

雇员

2 个答案: