心得

這題要show出沒有訂過東西的客戶，所以可以使用LEFT JOIN來關聯兩張表。

問題

Suppose that a website contains two tables, the Customers table and the Orders table. Write a SQL query to find all customers who never order anything.

Table: Customers.

+----+-------+
| Id | Name  |
+----+-------+
| 1  | Joe   |
| 2  | Henry |
| 3  | Sam   |
| 4  | Max   |
+----+-------+

Table: Orders.

+----+------------+
| Id | CustomerId |
+----+------------+
| 1  | 3          |
| 2  | 1          |
+----+------------+

Using the above tables as example, return the following:

+-----------+
| Customers |
+-----------+
| Henry     |
| Max       |
+-----------+

答案

1. LEFT JOIN

   # Write your MySQL query statement below
   SELECT      a.Name AS Customers
   FROM        Customers AS a
   LEFT JOIN   Orders AS b
   ON          a.Id = b.CustomerId
   WHERE       b.CustomerId IS NULL

2. Sub Query

   # Write your MySQL query statement below
   SELECT  a.Name AS Customers
   FROM    Customers AS a
   WHERE   (   SELECT  COUNT(1)
               FROM    Orders AS z
               WHERE   z.CustomerId = a.Id) = 0

心得：

找出該資料表內第二高薪水的筆數，若無則回傳null。

問題：

Write a SQL query to get the second highest salary from the Employee table.

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

For example, given the above Employee table, the second highest salary is 200. If there is no second highest salary, then the query should return null.

答案：

1. Sub Query

   # Write your MySQL query statement below
   SELECT  MAX(a.Salary) AS SecondHighestSalary
   FROM    Employee AS a
   WHERE   a.Salary < (SELECT  MAX(z.Salary)
                       FROM    Employee AS z)

2. DISTINCT > 不重複

   # Write your MySQL query statement below
   SELECT
   (
       SELECT DISTINCT Salary
       FROM            Employee
       ORDER BY        Salary DESC
       LIMIT           1, 1
   ) AS SecondHighestSalary

3. GROUP BY > 不重複

   # Write your MySQL query statement below
   SELECT
   (
       SELECT          Salary
       FROM            Employee
       GROUP BY        Salary
       ORDER BY        Salary DESC
       LIMIT           1, 1
   ) AS SecondHighestSalary

    

心得：

題目要求找出有哪些人的薪水比主管高，找出名字並修改欄位名稱為Employee。

題目：

The Employee table holds all employees including their managers. Every employee has an Id, and there is also a column for the manager Id.

+----+-------+--------+-----------+
| Id | Name  | Salary | ManagerId |
+----+-------+--------+-----------+
| 1  | Joe   | 70000  | 3         |
| 2  | Henry | 80000  | 4         |
| 3  | Sam   | 60000  | NULL      |
| 4  | Max   | 90000  | NULL      |
+----+-------+--------+-----------+

Given the Employee table, write a SQL query that finds out employees who earn more than their managers. For the above table, Joe is the only employee who earns more than his manager.

+----------+
| Employee |
+----------+
| Joe      |
+----------+

答案：

1. Sub Select

   # Write your MySQL query statement below
   SELECT  a.Name AS Employee
   FROM    Employee AS a
   WHERE   a.Salary > (SELECT  z.Salary 
                       FROM    Employee AS z
                       WHERE   z.Id = a.ManagerId);

2. Join

   # Write your MySQL query statement below
   SELECT  a.Name AS Employee
   FROM    Employee AS a
   JOIN    Employee AS b
   ON      a.ManagerId = b.Id
   WHERE   a.Salary > b.Salary

3. From

   # Write your MySQL query statement below
   SELECT  a.Name AS Employee
   FROM    Employee AS a, Employee AS b
   WHERE   a.ManagerId = b.Id
   AND     a.Salary > b.Salary