7天快速掌握MySQL-DAY6

--code--
CREATE TABLE Employee
 (
 Id INT NOT NULL PRIMARY KEY,
 Name VARCHAR(50)  NOT NULL,
 Salary INT NOT NULL,
 Departmentid  INT  NOT NULL
 )  ;
 
INSERT INTO Employee(Id,Name,Salary,Departmentid) VALUES (1,'Joe',70000,1);
INSERT INTO Employee(Id,Name,Salary,Departmentid) VALUES (2,'Henry',80000,2);
INSERT INTO Employee(Id,Name,Salary,Departmentid) VALUES (3,'Sam',60000,2);
INSERT INTO Employee(Id,Name,Salary,Departmentid) VALUES (4,'Max',90000,1);
 ---------------------------
CREATE TABLE Department ( 
Id INT NOT NULL PRIMARY KEY, 
Name VARCHAR(50)  NOT NULL );
 
INSERT INTO Department(Id,Name) VALUES (1,'IT');
INSERT INTO Department(Id,Name) VALUES (2,'Sales');
--------------------------------
mysql> SELECT
    -> b.Name AS Department,
    -> a.Name AS Employee,
    -> MAX(a.Salary) AS Salary
    -> FROM Employee AS a
    -> INNER JOIN Department AS b
    -> ON a.Departmentid = b.Id
    -> GROUP BY b.Name ;
+------------+----------+--------+
| Department | Employee | Salary |
+------------+----------+--------+
| IT         | Joe      |  90000 |
| Sales      | Henry    |  80000 |
+------------+----------+--------+
2 rows in set (0.00 sec)

mysql> CREATE TABLE seat
    -> (
    -> id INT NOT NULL ,
    -> student VARCHAR(50) NOT NULL
    -> );
Query OK, 0 rows affected (0.02 sec)

mysql> INSERT INTO seat(id,student) VALUES (1,'Abbot');
Query OK, 1 row affected (0.01 sec)

mysql> INSERT INTO seat(id,student) VALUES (2,'Doris');
Query OK, 1 row affected (0.00 sec)

mysql> INSERT INTO seat(id,student) VALUES (3,'Emerson');
Query OK, 1 row affected (0.00 sec)

mysql> INSERT INTO seat(id,student) VALUES (4,'Green');
Query OK, 1 row affected (0.00 sec)

mysql> INSERT INTO seat(id,student) VALUES (5,'Jeames');
Query OK, 1 row affected (0.00 sec)
--找出最大值，判断奇偶
mysql> select max(id) as max_id from seat;
+--------+
| max_id |
+--------+
|      5 |
+--------+
1 row in set (0.00 sec)


mysql> update seat set id =
    -> case
    -> when id = 5 then 5 # 最大id奇数，不变
    -> when mod (id, 2) = 1 then id + 1
    -> else id -1
    -> end;
Query OK, 4 rows affected (0.01 sec)
Rows matched: 5  Changed: 4  Warnings: 0

mysql> select * from seat;
+----+---------+
| id | student |
+----+---------+
|  2 | Abbot   |
|  1 | Doris   |
|  4 | Emerson |
|  3 | Green   |
|  5 | Jeames  |
+----+---------+
5 rows in set (0.00 sec)

Trips 表中存所有出租车的行程信息。每段行程有唯一键 Id，Client_Id 和 Driver_Id 是 Users 表中 
Users_Id 的外键。Status 是枚举类型，枚举成员为
 (‘completed’, ‘cancelled_by_driver’, ‘cancelled_by_client’)。
+----+-----------+-----------+---------+--------------------+----------+
| Id | Client_Id | Driver_Id | City_Id |        Status      |Request_at|
+----+-----------+-----------+---------+--------------------+----------+
| 1  |     1     |    10     |    1    |     completed      |2013-10-01|
| 2  |     2     |    11     |    1    | cancelled_by_driver|2013-10-01|
| 3  |     3     |    12     |    6    |     completed      |2013-10-01|
| 4  |     4     |    13     |    6    | cancelled_by_client|2013-10-01|
| 5  |     1     |    10     |    1    |     completed      |2013-10-02|
| 6  |     2     |    11     |    6    |     completed      |2013-10-02|
| 7  |     3     |    12     |    6    |     completed      |2013-10-02|
| 8  |     2     |    12     |    12   |     completed      |2013-10-03|
| 9  |     3     |    10     |    12   |     completed      |2013-10-03| 
| 10 |     4     |    13     |    12   | cancelled_by_driver|2013-10-03|
+----+-----------+-----------+---------+--------------------+----------+
Users 表存所有用户。每个用户有唯一键 Users_Id。Banned 表示这个用户是否被禁止，Role 则是一个表示
（‘client’, ‘driver’, ‘partner’）的枚举类型。
+----------+--------+--------+
| Users_Id | Banned |  Role  |
+----------+--------+--------+
|    1     |   No   | client |
|    2     |   Yes  | client |
|    3     |   No   | client |
|    4     |   No   | client |
|    10    |   No   | driver |
|    11    |   No   | driver |
|    12    |   No   | driver |
|    13    |   No   | driver |
+----------+--------+--------+
写一段 SQL 语句查出 2013年10月1日 至 2013年10月3日 期间非禁止用户的取消率。基于上表，你的 SQL 语句
应返回如下结果，取消率（Cancellation Rate）保留两位小数。
+------------+-------------------+
|     Day    | Cancellation Rate |
+------------+-------------------+
| 2013-10-01 |       0.33        |
| 2013-10-02 |       0.00        |
| 2013-10-03 |       0.50        |
+------------+-------------------+

将昨天employee表清空，重新插入以下数据（其实是多插入5,6两行）：
+----+-------+--------+--------------+
| Id | Name  | Salary | DepartmentId |
+----+-------+--------+--------------+
| 1  | Joe   | 70000  | 1            |
| 2  | Henry | 80000  | 2            |
| 3  | Sam   | 60000  | 2            |
| 4  | Max   | 90000  | 1            |
| 5  | Janet | 69000  | 1            |
| 6  | Randy | 85000  | 1            |
+----+-------+--------+--------------+
编写一个 SQL 查询，找出每个部门工资前三高的员工。例如，根据上述给定的表格，查询结果应返回：
+------------+----------+--------+
| Department | Employee | Salary |
+------------+----------+--------+
| IT         | Max      | 90000  |
| IT         | Randy    | 85000  |
| IT         | Joe      | 70000  |
| Sales      | Henry    | 80000  |
| Sales      | Sam      | 60000  |
+------------+----------+--------+

此外，请考虑实现各部门前N高工资的员工功能。

依然是昨天的分数表，实现排名功能，但是排名是非连续的，如下：
+-------+------+
| Score | Rank |
+-------+------+
| 4.00  | 1    |
| 4.00  | 1    |
| 3.85  | 3    |
| 3.65  | 4    |
| 3.65  | 4    |
| 3.50  | 6    |
+-------+------+