601. 体育馆的人流量

X 市建了一个新的体育馆，每日人流量信息被记录在这三列信息中：序号 (id)、日期 (date)、 人流量 (people)。

请编写一个查询语句，找出高峰期时段，要求连续三天及以上，并且每天人流量均不少于100。

例如，表 stadium：

+------+------------+-----------+
| id   | date       | people    |
+------+------------+-----------+
| 1    | 2017-01-01 | 10        |
| 2    | 2017-01-02 | 109       |
| 3    | 2017-01-03 | 150       |
| 4    | 2017-01-04 | 99        |
| 5    | 2017-01-05 | 145       |
| 6    | 2017-01-06 | 1455      |
| 7    | 2017-01-07 | 199       |
| 8    | 2017-01-08 | 188       |
+------+------------+-----------+

对于上面的示例数据，输出为：

+------+------------+-----------+
| id   | date       | people    |
+------+------------+-----------+
| 5    | 2017-01-05 | 145       |
| 6    | 2017-01-06 | 1455      |
| 7    | 2017-01-07 | 199       |
| 8    | 2017-01-08 | 188       |
+------+------------+-----------+

Note:
每天只有一行记录，日期随着 id 的增加而增加。

思路/笔记：自连接的应用，这个语句其实有点取巧，因为日期都是连续的，而Id属于INT类型，比TO_DATE(date, ‘yyyymmdd’)/TO_CHAR(DATE, ‘yyyymmdd’)这样的运算快多了。

select distinct t1.*
  from stadium t1, stadium t2, stadium t3
 where t1.people >= 100 and t2.people >= 100 and t3.people >= 100
   and ((t1.id - t2.id = 1 and t1.id - t3.id = 2 and t2.id - t3.id =1)
        or (t2.id - t1.id = 1 and t2.id - t3.id = 2 and t1.id - t3.id =1)
        or (t3.id - t2.id = 1 and t2.id - t1.id =1 and t3.id - t1.id = 2)
       )
 order by t1.id

262. 行程和用户

Trips 表中存所有出租车的行程信息。每段行程有唯一健 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        |
+------------+-------------------+

思路/笔记：常用操作，手动滑稽

select t.Request_at AS "Day",round(sum(case when t.Status = 'completed' then 0
                                           else 1
                                      end) / count(0),2) AS "Cancellation Rate"
 from trips t
where EXISTS (select u.Users_Id
                from users u
               where u.Users_Id = t.Client_Id
                 and u.Banned = 'No')
  and t.Request_at between '2013-10-01' and '2013-10-03'
group by Day

185. 部门工资前三高的员工

Employee 表包含所有员工信息，每个员工有其对应的 Id, salary 和 department Id 。

+----+-------+--------+--------------+
| 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            |
+----+-------+--------+--------------+

Department 表包含公司所有部门的信息。

+----+----------+
| Id | Name     |
+----+----------+
| 1  | IT       |
| 2  | Sales    |
+----+----------+

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  |
+------------+----------+--------+

思路/笔记：

1. 通过自连接排定次序
2. join部门表

select d.Name Department, t.Name Employee, Salary
  from (select Name,Salary,DepartmentId,
               (select count(distinct e1.Salary)
                  from Employee e1
                 where e1.DepartmentId = e2.DepartmentId
                   and e1.Salary > e2.Salary) rank
          from Employee e2)t
  left join Department d on d.Id = t.DepartmentId
 where rank < 3
   and d.Id is not null
 order by DepartmentId,Salary desc

总结

数据库操作的效率差别，需要在百万/千万/亿/十亿级别，才会有明显的差异，比如BAT的数据级别，只能自行研发数据库。所以这里只涉及到了很多基础的操作，比如case的用法，自连接的用法等。具体在项目常用到的操作有根据范式来设计表、根据常用到的字段来建立索引、考虑根据数据量按日/周/月分区、如何通过存储过程/函数高效转移数据、如何设置UNDO表空间保证数据库性能、怎样查看常用视图与统计表、如何备份、还原与清理数据…都还是需要在实际项目里去学习。