LeetCode Database题解

175. Combine Two Tables

使用外连接即可。

# Write your MySQL query statement below
select FirstName, LastName, City, State from Person left outer join Address on Person.PersonId = Address.PersonId;

# Write your MySQL query statement below
select p.FirstName, p.LastName, a.City, a.State from Person as p left outer join Address as a on p.PersonId = a.PersonId;

176. Second Highest Salary

查询第二高工资值，可能无此值。使用聚集函数与子查询即可，注意MAX可能返回NULL。

# Write your MySQL query statement below
select max(Salary) as SecondHighestSalary from Employee where Salary < (select max(Salary) from Employee);

# Write your MySQL query statement below
select max(Salary) as SecondHighestSalary from Employee where Salary not in(select max(Salary) from Employee);

177. Nth Highest Salary

查询第N高工资值，可能无此值。使用IFNULL，然后再对结果进行限制即可。

CREATE FUNCTION getNthHighestSalary(N INT) RETURNS INT
BEGIN
  DECLARE M INT;
  SET M = N - 1;
  RETURN (
      # Write your MySQL query statement below.
      SELECT IFNULL((SELECT DISTINCT Salary FROM Employee ORDER BY Salary DESC LIMIT M, 1), NULL)
  );
END

178. Rank Scores

按得分排名并计算排名。使用子查询、聚集函数，然后再对结果进行排序即可。

# Write your MySQL query statement below
select Scores.Score, (select count(*) from (select distinct Score from Scores) as DistinctScores where DistinctScores.Score >= Scores.Score) as Rank from Scores order by Score desc;

# Write your MySQL query statement below
select Scores.Score, count(DistinctScores.Score) as Rank from Scores, (select distinct Score from Scores) as DistinctScores where Scores.Score <= DistinctScores.Score group by Scores.Id, Scores.Score order by Scores.Score desc;

180. Consecutive Numbers

找出至少连续出现3次的Num。先排序，再查询即可。也有人使用变量来解答。

# Write your MySQL query statement below
select distinct(l1.Num) as ConsecutiveNums from Logs l1, Logs l2, Logs l3 where l1.Id + 1 = l2.Id and l1.Id + 2 = l3.Id and l1.Num = l2.Num and l1.Num = l3.Num;

181. Employees Earning More Than Their Managers

查询工资比上司高的员工。使用自连接即可。

# Write your MySQL query statement below
select e1.Name as Employee from Employee as e1, Employee as e2 where e1.ManagerId = e2.Id and e1.Salary > e2.Salary;

182. Duplicate Emails

查找表中有重复的emails。使用数据分组或自连接即可。

# Write your MySQL query statement below
select Email from Person group by Email having count(*) > 1;

# Write your MySQL query statement below
select distinct(p1.Email) from Person as p1, Person as p2 where p1.Id != p2.Id and p1.Email = p2.Email;

183. Customers Who Never Order

查出没有订单的客户。使用数据过滤即可。

# Write your MySQL query statement below
select Name as Customers from Customers where Customers.Id not in (select distinct(CustomerId) from Orders);

184. Department Highest Salary

查询每个部内最高的工资，最高工资可能有多个员工。这题相对来说稍复杂，使用子查询与数据分组来解决。

# Write your MySQL query statement below
select Department.Name as Department, Employee.Name as Employee, max(Salary) as Salary from Employee, Department where Employee.DepartmentId = Department.Id;
# 错误解法，结果会出现[null, null, null]这种情况

# Write your MySQL query statement below
select Department.Name as Department, Employee.Name as Employee, Salary from Employee, Department where Employee.DepartmentId = Department.Id order by Salary limit 1;
# 错误解法，结果只会有一行

# Write your MySQL query statement below
select d.Name as Department, e.Name as Employee, e.Salary as Salary 
from Department as d, (select Name, max(Salary) as Salary, DepartmentId from Employee group by DepartmentId) as e 
where d.Id = e.DepartmentId;
# 错误解法，当同一部门有两个最高的Salary时候，结果只有一行，故必须使用下面解法，使用2个Employee表

# Write your MySQL query statement below
select d.Name as Department, e.Name as Employee, e.Salary as Salary 
from Department as d, Employee as e, (select max(Salary) as Salary,  DepartmentId from Employee group by DepartmentId) as h
where d.Id = h.DepartmentId and e.Salary = h.Salary and e.DepartmentId = h.DepartmentId;