LeetCode SQL刷题

1、表联合
Table1: Person (PersonId is the primary key column for this table.)

Column Name	Type
PersonId	int
FirstName	varchar
LastName	varchar

Table2: Address (AddressId is the primary key column for this table.)

Column Name	Type
AddressId	int
PersonId	int
City	varchar
State	varchar

用这两张表输出 FirstName, LastName, City, State四个字段，无论PersonId是否为空。

select person.FirstName,person.LastName,Address.City,Address.State 
from Person left join Address
on Person.PersonId = Address.PersonId

2、 Second Highest Salary
table:employee

Id	Salary
1	100
2	200
3	300

question: give the second highest salary.If there is no second highest salary, then the query should return null.
求排名第二的工资，如果没有，返回NULL。

##用LIMIT限定数据记录查询数量
SELECT DISTINCT Salary  AS SecondHighestSalary
FROM Employee
ORDER BY Salary
LIMIT 1,1

以上写法没有考虑到 不存在第二位的情况，例如只有一条数据记录。这时把以上查询的结果作为临时表。

SELECT *
FROM(
SELECT DISTINCT Salary  
FROM Employee
ORDER BY Salary
LIMIT 1,1) AS SecondHighestSalary

另一种解法：

SELECT MAX(Salary) as SecondHighestSalary FROM Employee 
WHERE Salary NOT IN(SELECT MAX(Salary) from Employee)

3、Department Highest Salary

table1：The Employee table holds all employees

Id	Name	Salary	DepartmentId
1	Joe	70000	1
2	Henry	80000	2
3	Sam	60000	2
4	Max	90000	1

table2：The Department table holds all departments of the company.

Id	Name
1	IT
2	Sales

查找每个部门的最高工资，如下：

Department	Employee	Salary
IT	Max	90000
Sales	Henry	80000

SELECT  Department.Name AS Department,A.Name AS Employee,A.Salary AS Salary
FROM
(SELECT Name, MAX(Salary) AS Salary,DepartmentId
FROM employee GROUP BY DepartmentId) A
LEFT JOIN Department 
ON A.DepartmentId = Department.Id

结果出错：
SELECT Name, MAX(Salary) AS Salary,DepartmentId
FROM employee GROUP BY DepartmentId
 按照DepartmentId分组后，每组如果有多个员工有相同的最高工资，而MAX只返回一个员工Name。

1.先联合两表，找出各个部门的最高薪水
SELECT  Department.Id, Department.Name, MAX(Salary) AS Salary
FROM employee JOIN Department
ON employee.DepartmentId = Department.Id
GROUP BY Department.Id

2. 与雇员表联合，匹配出该工资的员工，这样就包括每组有多个员工有相同的最高工资的情况。
SELECT t.Name AS Department, e.Name AS Employee, t.Salary
FROM
(SELECT Department.Id, 
        Department.Name AS Name,
        MAX(Salary) AS Salary
FROM Employee JOIN Department
ON Employee.DepartmentId = Department.Id
GROUP BY Department.Id
) t 
JOIN Employee  e
ON t.Id=e.DepartmentId and t.Salary=e.Salary

猜你喜欢