刷题笔记（Mysql）

175. 组合两个表

表1: Person
+-------------+---------+
| 列名 | 类型 |
+-------------+---------+
| PersonId | int |
| FirstName | varchar |
| LastName | varchar |
+-------------+---------+

表2: Address
+--------------+---------+
| 列名 | 类型 |
+--------------+---------+
| AddressId | int |
| PersonId | int |
| City | varchar |
| State | varchar |
+-------------+---------+

编写一个 SQL 查询，满足条件：无论 person 是否有地址信息，都需要基于上述两表提供 person 的以下信息：FirstName, LastName, City, State

select A.FirstName, A.LastName, B.City, B.State
from Person A
left join (select distinct PersonId, City, State from Address) B
on A.PersonId=B.PersonId;

176. 第二高的薪水

编写一个 SQL 查询，获取 Employee 表中第二高的薪水（Salary） 。

+----+--------+
| Id | Salary |
+----+--------+
| 1 | 100 |
| 2 | 200 |
| 3 | 300 |
+----+--------+

select IFNULL((select distinct(Salary) from Employee order by Salary desc limit 1,1),null) 
as SecondHighestSalary

177. 第N高的薪水

编写一个 SQL 查询，获取 Employee 表中第 n 高的薪水（Salary）。

+----+--------+
| Id | Salary |
+----+--------+
| 1 | 100 |
| 2 | 200 |
| 3 | 300 |
+----+--------+
例如上述 Employee 表，n = 2 时，应返回第二高的薪水 200。如果不存在第 n 高的薪水，那么查询应返回 null。

+------------------------+
| getNthHighestSalary(2) |
+------------------------+
| 200 |
+------------------------+

CREATE FUNCTION getNthHighestSalary(N INT) RETURNS INT
BEGIN
  DECLARE c INT default if(N>0,N-1,1);
  RETURN (
      # Write your MySQL query statement below.
      select Salary from Employee group by Salary order by Salary desc limit c,1
  );
END