[英]Is there a way to make this SQL more efficient?
請考慮以下表格:
deptid (type:INT)
deptname (type: TEXT)
hours (type:INT)
active (type:BIT)
empid (type:INT)
empname (type: TEXT)
deptid (type: INT)
designation (type: TEXT)
salary (type: INT)
編寫查詢以返回總人數為4或更多的那些部門的雇員的empname和deptname列。 記錄應按姓氏的字母順序返回
這是我的看法:
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;
您將如何改進?
這比較短,而且執行速度可能也更快
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;
從分組開始。 您不需要內部查詢中的COUNT。 然后,聯接兩個表只是為了獲得名稱。
之所以使用INNER JOIN,是因為一旦計數完成,我們已經知道
試試這個查詢,它將正常工作。
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
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