Leetcode - MySQL问题汇总

176. Second Highest Salary

ifnull函数，ifnull(n1, n2) — 如果n1≠null，返回n1， 如果n1=null，返回null

select ifnull(
    (select distinct salary
    from employee
    order by salary desc
    limit 1 offset 1), NULL) as SecondHighestSalary

177. Nth Highest Salary

CREATE FUNCTION getNthHighestSalary(N INT) RETURNS INT
BEGIN
declare m int;
set m=n-1;
  RETURN (
      select ifnull(
        (select distinct salary 
         from employee
         order by salary desc
         limit 1 offset m), NULL) as getNthHighestSalary
  );
END

• 如果直接用n-1会报错
  要设m=n-1 声明用declare
• 需要distinct() 才有null

178. Rank Scores

select score as Score, (select count(distinct score) from scores where score >= s.score) as Rank
from scores as s
order by score desc

Score	Rank
4.00	1
4.00	1
3.85	2
3.65	3
3.65	3
3.50	4