1、表聯合
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 |
用這兩張表輸出 FirstName, LastName, City, State四個字段，無論PersonId是否爲空。
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.
求排名第二的工資，如果沒有，返回NULL。
##用LIMIT限定數據記錄查詢數量
SELECT DISTINCT Salary AS SecondHighestSalary
FROM Employee
ORDER BY Salary
LIMIT 1,1
以上寫法沒有考慮到 不存在第二位的情況，例如只有一條數據記錄。這時把以上查詢的結果作爲臨時表。
SELECT *
FROM(
SELECT DISTINCT Salary
FROM Employee
ORDER BY Salary
LIMIT 1,1) AS SecondHighestSalary另一種解法：
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 |
查找每個部門的最高工資，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
結果出錯：
SELECT Name, MAX(Salary) AS Salary,DepartmentId
FROM employee GROUP BY DepartmentId
按照DepartmentId分組後，每組如果有多個員工有相同的最高工資，而MAX只返回一個員工Name。
1.先聯合兩表，找出各個部門的最高薪水
SELECT Department.Id, Department.Name, MAX(Salary) AS Salary
FROM employee JOIN Department
ON employee.DepartmentId = Department.Id
GROUP BY Department.Id
2. 與僱員表聯合，匹配出該工資的員工，這樣就包括每組有多個員工有相同的最高工資的情況。
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