LeetCode 刷提記錄 SQL

177. nth highest salary

create function getnthhighestsalary(n int) returns int
begin
declare m int; #宣告區域性變數
set m = n-1; #賦值，因為limit 後面的引數不能是乙個表示式
return (
# write your mysql query statement below.
select distinct salary 
from employee
order by salary desc
limit m,1
);end

因為這個題目需要完成的只是定義函式，所以可以不用ifnull()或者coalesce()來判斷返回值是不是非空，如果返回空值，在呼叫函式時再解決

178

. rank scores

# write your mysql query statement below
select c.score as score, r as rank 
from scores c join (select a.score as score, @i:=@i+1 as r
from (select distinct score from scores) as a,(select @i:=0) as b
order by a.score desc) as t 
on c.score = t.score
order by c.score desc;

思路是先把去重後的成績排序生成新錶t，然後將原始表c 和新錶t 根據對應成績聯接，則相同的成績就會有相同的排序

注意生成排序的方法是定義了乙個自增的變數，然後用select 賦值為0（select 賦值時要用 :=）

其他方法：（比較常見的方法）

# write your mysql query statement below select a.score as score, count(distinct b.score) as rank from scores a,scores b where a.score<=b.score group by a.id,a.score

order by a.score desc

重點是a.score<=b.score

這裡，通過計算去重後的大於等於a.score的資料的個數來給a.score排序

180

. consecutive numbers

select c.num as consecutivenums
from logs a join logs b on b.id = (a.id+1) and b.num = a.num
join logs c on c.id = (b.id+1) and c.num = b.num
group by c.num

注意方法

184

. department highest salary

select d.name as department, e.name as employee, e.salary as salary
from employee e join department d on e.departmentid = d.id
where e.salary in (select max(a.salary)
from employee a 
where a.departmentid = e.departmentid
)

注意不能直接查詢

d.name,e.name,max(salary).......group by d.name，因為group by 只返回分組後的一條資料，如果這樣做，會預設取查詢到的第乙個e.name，而不是max(salary) 對應的 e.name

185. department top three salaries

# write your mysql query statement below select d.name as department, e.name as employee, e.salary as salary from employee e join department d on e.departmentid = d.id where e.salary >=coalesce ((select distinct a.salary from employee a where e.departmentid = a.departmentid order by a.salary desc limit 2,1),0) # mysql中 in 和 limit 不能連用

order by d.id,e.salary desc

思路是找出每個部門排第三的薪水對應的記錄，然後查詢》=這個薪水的記錄

注意mysql中，limit和in 不能連用，但是和比較運算子可以一塊用，此外在一些問題中還可以用建個臨時表的方法來解決limit和in不能連用的問題

LeetCode 刷提記錄 SQL

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LeetCode 刷提記錄 SQL

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