Leetcode 6.22 다시 풀기 180,181,182,183,184 +성능 고민

180. Consecutive Numbers

SELECT a.num as ConsecutiveNums
FROM 
        (  
        SELECT id
              ,num
              ,(LEAD(num,1) OVER (ORDER BY id))l
              ,(LEAD(num,2) OVER (ORDER BY id))m
        FROM logs
        )a
WHERE a.num=a.l=a.m

mysql로 했을 때는 WHERE a.num=a.l=a.m 이게 되는데 mssql로 하니까 WHERE a.num=a.l=a.m 이게 에러가 난다.

왜 그럴까? 곧 처리하기!

181. Employees Earning More Than Their Managers

SELECT e.name as Employee
FROM Employee e
    JOIN Employee e2 ON e.managerid=e2.id
WHERE e.salary > e2.salary

- 오타도 실력이다! 

- more than 은 영어에서 초과!

182. Duplicate Emails

SELECT email 
FROM Person
GROUP BY email
HAVING COUNT(*)>1

183. Customers Who Never Order

SELECT c.name as Customers
FROM Customers c
    LEFT JOIN Orders o ON c.id=o.customerid
WHERE customerid IS NULL

Runtime: 876 ms, faster than 10.91% of MySQL online submissions for Customers Who Never Order.

SELECT name as Customers
FROM Customers
WHERE id NOT IN ( SELECT customerid FROM Orders )

Runtime: 456 ms, faster than 74.53% of MySQL online submissions for Customers Who Never Order.

성능을 높이기 위해 여러가지 방법을 시도해 보고 있는데.. 의문 덩어리가 남았다. 

왜 같은 문제, 같은 쿼리인데 성능이 천차만별 인 것일까?

그리고 이 두개는 왜 다른 것일까? ㅎㅎㅎㅎ

그래서 질문을 남겼다. 

- 지엽적인 문제일 수 있으니 일단 저장해 놓고 가겠다. 

184. Department Highest Salary

SELECT d.name as Department
      ,e.name as Employee
      ,e.salary 
FROM Employee e
     LEFT JOIN Department d ON e.departmentid=d.id
WHERE (e.departmentid,e.salary) IN (
                                    SELECT departmentid
                                    ,MAX(salary) as salary
                                     FROM Employee  
                                     GROUP BY departmentid 
                                     )

- WHERE ( , ) IN ( ) 이 핵심 

(다른 풀이) 

SELECT D.NAME AS DEPARTMENT,
E.NAME AS EMPLOYEE,
E.SALARY
FROM EMPLOYEE E
JOIN DEPARTMENT D ON 
E.DEPARTMENTID = D.ID
WHERE SALARY = (SELECT MAX(SALARY) FROM EMPLOYEE WHERE D.ID = EMPLOYEE.DEPARTMENTID);

이런 방법도 있군! 서브 쿼리에 where 조건을 넣어 주는 것