LeetCode 数据库练习笔记
175. 组合两表
左查询： left join ... on ...
176. 第二高的薪水 如果不存在第二高的薪水，则返回null (考察 ifnull 和 limit/offset)
DISTINCT: 去重
SQL查询语句中的 limit 与 offset 的区别：
limit y 分句表示: 读取 y 条数据
limit x, y 分句表示: 跳过 x 条数据，读取 y 条数据
limit y offset x 分句表示: 跳过 x 条数据，读取 y 条数据
查询第N高数据？
题解一： 查询出最高的数据，在此前提上，再查出最高的数据
题解二： 使用limit和offset 查询
ifnull:
select ifnull(
(select distinct Salary from Employee order by Salary desc limit 1,1),null)
as "SecondHighestSalary";
窗口函数：
select distinct Salary from
(
select Salary,dense_rank() over(order by Salary desc) rnk from Employee
) t where rnk=@N
181.超过经理收入的员工 (考察 内连接)
Employee表
+----+-------+--------+-----------+
| Id | Name | Salary | ManagerId |
+----+-------+--------+-----------+
| 1 | Joe | 70000 | 3 |
| 2 | Henry | 80000 | 4 |
| 3 | Sam | 60000 | NULL |
| 4 | Max | 90000 | NULL |
+----+-------+--------+-----------+
预期效果
+----------+
| Employee |
+----------+
| Joe |
+----------+
# Write your MySQL query statement below
# 子查询
#select Name as Employee from Employee a
# where Salary >(Select Salary from Employee where Id = a.ManagerId);
#使用where语句 会产生笛卡尔积 不推荐使用 消耗内存比较大
#select a.Name as 'Employee'
#from Employee a , Employee b
#where a.ManagerId = b.Id
#and a.Salary > b.Salary;
#使用join 方法 优化
select a.Name as 'Employee'
from Employee a join Employee b
on a.ManagerId = b.Id
and a.Salary > b.Salary;
182. 查找重复的电子邮箱 (考察 group by) 优先级顺序：where>group by>having>order by
person表
+----+---------+
| Id | Email |
+----+---------+
| 1 | [email protected] |
| 2 | [email protected] |
| 3 | [email protected] |
+----+---------+
预期效果
+---------+
| Email |
+---------+
| [email protected] |
+---------+
# Write your MySQL query statement below
# 自连接 表数据庞大的时候 查询相对慢
#select distinct p1.Email Email
#from Person p1 join Person p2
#on p1.Email = p2.Email;
# group by having
#select Email from Person group by Email having count(Email)>1;
# group by 和 临时表
select Email from (select Email ,count(Email) as num from Person group by Email) as NemPerson where num > 1;
183. 从不订购的客户 (考察: not in )
Customers 表：
+----+-------+
| Id | Name |
+----+-------+
| 1 | Joe |
| 2 | Henry |
| 3 | Sam |
| 4 | Max |
+----+-------+
Orders 表：
+----+------------+
| Id | CustomerId |
+----+------------+
| 1 | 3 |
| 2 | 1 |
+----+------------+
例如给定上述表格，你的查询应返回：
+-----------+
| Customers |
+-----------+
| Henry |
| Max |
+-----------+
# Write your MySQL query statement below
#自连接 80 ms 一般解
#select c.Name Customers from Customers c left join
#(select CustomerId , count(CustomerId) as num from Orders group by CustomerId)
#as o
#on c.Id = o.CustomerId
#where o.num is null;
# 优质解
#select c.Name Customers from Customers c left join Orders o on c.Id = o.CustomerId #where o.CustomerId is null;
#官方解 最优解
select customers.name Customers from customers
where customers.id not in
(
select CustomerId from Orders
);
196. 删除重复的电子邮箱 (考察 Delete Delete和 自连接 left/right join ... on ...不能同时使用 常与where 使用 )
编写一个 SQL 查询，来删除 Person 表中所有重复的电子邮箱，重复的邮箱里只保留 Id 最小 的那个。
+----+------------------+
| Id | Email |
+----+------------------+
| 1 | [email protected] |
| 2 | [email protected] |
| 3 | [email protected] |
+----+------------------+
Id 是这个表的主键
例如，在运行你的查询语句之后，上面的 Person 表应返回以下几行:
+----+------------------+
| Id | Email |
+----+------------------+
| 1 | [email protected] |
| 2 | [email protected] |
+----+------------------+
# Write your MySQL query statement below
#delete p1 from Person p1 , Person p2 where p1.Email = p2.Email and p1.Id > p2.Id;
#1.找出要删除的重复数据
#select distinct p.Id as id from person p , person s where p.Email = s.Email and p.Id>s.Id;
#2.MySQL不允许select 和 delete 操作同一个表
#select id from (select distinct p.Id as id from person p , person s where p.Email = s.Email and p.Id>s.Id) as tmp;
#3.删除重复数据
delete from person where id in ( select id from (select distinct p.Id as id from person p , person s where p.Email = s.Email and p.Id>s.Id) as tmp);
#91ms 没有考虑去重
#delete p1 from person p1, person p2 where p1.Email = p2.Email and p1.Id > p2.Id;
197.上升的温度 ( DATEDIFF() 函数计算两个日期之间的间隔天数 )
给定一个 Weather 表，编写一个 SQL 查询，来查找与之前（昨天的）日期相比温度更高的所有日期的 Id。
+---------+------------------+------------------+
| Id(INT) | RecordDate(DATE) | Temperature(INT) |
+---------+------------------+------------------+
| 1 | 2015-01-01 | 10 |
| 2 | 2015-01-02 | 25 |
| 3 | 2015-01-03 | 20 |
| 4 | 2015-01-04 | 30 |
+---------+------------------+------------------+
例如，根据上述给定的 Weather 表格，返回如下 Id:
+----+
| Id |
+----+
| 2 |
| 4 |
+----+
# Write your MySQL query statement below
#select w1.Id from Weather w1,Weather w2 where w1.Temperature > w2.Temperature and datediff(w1.RecordDate,w2.RecordDate)=1;
595. 大的国家 (考察:or 的使用方法 另外的解法 :union /union all UNION 操作符用于合并两个或多个 SELECT 语句的结果集。)
这里有张 World 表
+-----------------+------------+------------+--------------+---------------+
| name | continent | area | population | gdp |
+-----------------+------------+------------+--------------+---------------+
| Afghanistan | Asia | 652230 | 25500100 | 20343000 |
| Albania | Europe | 28748 | 2831741 | 12960000 |
| Algeria | Africa | 2381741 | 37100000 | 188681000 |
| Andorra | Europe | 468 | 78115 | 3712000 |
| Angola | Africa | 1246700 | 20609294 | 100990000 |
+-----------------+------------+------------+--------------+---------------+
如果一个国家的面积超过300万平方公里，或者人口超过2500万，那么这个国家就是大国家。
编写一个SQL查询，输出表中所有大国家的名称、人口和面积。
例如，根据上表，我们应该输出:
+--------------+-------------+--------------+
| name | population | area |
+--------------+-------------+--------------+
| Afghanistan | 25500100 | 652230 |
| Algeria | 37100000 | 2381741 |
+--------------+-------------+--------------+
# Write your MySQL query statement below
select name , population , area from World where area>3000000 or population >25000000;
select name,population,area from World where area > 3000000 union select name,population,area from World where population > 25000000;
596. 超过5名学生的课 (grouo by && having || group by && 子查询) where 要比 having 速度快很多
有一个courses 表 ，有: student (学生) 和 class (课程)。
请列出所有超过或等于5名学生的课。
例如,表:
+---------+------------+
| student | class |
+---------+------------+
| A | Math |
| B | English |
| C | Math |
| D | Biology |
| E | Math |
| F | Computer |
| G | Math |
| H | Math |
| I | Math |
+---------+------------+
应该输出:
+---------+
| class |
+---------+
| Math |
+---------+
Note:
学生在每个课中不应被重复计算。
# Write your MySQL query statement below
#group by && having 速度慢
#select class from courses group by class having count(distinct student)>=5 ;
#group by && 子查询 where 速度快
#select class, count(class) as num from courses group by class ;
select class from (select class, count(distinct student) as num from courses group by class ) as tmp where num >=5;
620. 有趣的电影 (考察 按顺序排列: order by lke not like 判断奇偶性 mod(id,2)=1; description可用 本题效率不高)
not like 和 != 效率相比 !=效率要比 not like 速率高 精确查找用!= 模糊查找用 like / not like
某城市开了一家新的电影院，吸引了很多人过来看电影。该电影院特别注意用户体验，专门有个 LED显示板做电影推荐，上面公布着影评和相关电影描述。
作为该电影院的信息部主管，您需要编写一个 SQL查询，找出所有影片描述为非 boring (不无聊) 的并且 id 为奇数 的影片，结果请按等级 rating 排列。
例如，下表 cinema:
+---------+-----------+--------------+-----------+
| id | movie | description | rating |
+---------+-----------+--------------+-----------+
| 1 | War | great 3D | 8.9 |
| 2 | Science | fiction | 8.5 |
| 3 | irish | boring | 6.2 |
| 4 | Ice song | Fantacy | 8.6 |
| 5 | House card| Interesting| 9.1 |
+---------+-----------+--------------+-----------+
对于上面的例子，则正确的输出是为：
+---------+-----------+--------------+-----------+
| id | movie | description | rating |
+---------+-----------+--------------+-----------+
| 5 | House card| Interesting| 9.1 |
| 1 | War | great 3D | 8.9 |
+---------+-----------+--------------+-----------+
# Write your MySQL query statement below
#按等级排 order by rating desc
#非boring description!= boring
#id为奇数 id%2=1
#select * from cinema where id % 2 and description <> 'boring' order by rating desc;
#select id , movie , description , rating from cinema where description not like '%boring%' and mod(id,2)=1 order by rating DESC;
select id , movie , description , rating from cinema where description != '%boring%' and mod(id,2)=1 order by rating DESC;
627. 交换工资
给定一个 salary 表，如下所示，有 m = 男性 和 f = 女性 的值。交换所有的 f 和 m 值（例如，将所有 f 值更改为 m，反之亦然）。要求只使用一个更新（Update）语句，并且没有中间的临时表。
注意，您必只能写一个 Update 语句，请不要编写任何 Select 语句。
例如：
| id | name | sex | salary |
|----|------|-----|--------|
| 1 | A | m | 2500 |
| 2 | B | f | 1500 |
| 3 | C | m | 5500 |
| 4 | D | f | 500 |
运行你所编写的更新语句之后，将会得到以下表:
| id | name | sex | salary |
|----|------|-----|--------|
| 1 | A | f | 2500 |
| 2 | B | m | 1500 |
| 3 | C | f | 5500 |
| 4 | D | m | 500 |
1.用if，注意中间是一个表达式 比如sex='m'
可以使用 if 函数判断 列的值 进行数据交换 当没有where 条件的时候会更新全表
IF 函数用法
IF(expr,v1,v2) 如果表达式 expr 成立，返回结果 v1；否则，返回结果 v2
update salary
set sex=if(sex='m','f','m')
2.用case when
因为本身的格式是
update 表 set 某个值= xxx
而case的写法
case 某个值 when 1 then 2
when 2 then 1
else 3 剩下来的写3
end
那下面的写法如果是f 则是m
如果是m （直接省略为else）则是f
因为这里是动态变化所以要用case或者if来写
update salary
set sex=(case sex
when 'f' then 'm'
else 'f'
end)
# Write your MySQL query statement below
#118 ms 91.54
#update salary set sex = case sex when 'm' then 'f' else 'm' end;
# 119 ms 87.75
update salary set sex = IF(sex = 'm', 'f', 'm');
#orcale中会用到
#UPDATE salary SET sex = DECODE(sex,'m','f','m');
1179. 重新格式化部门表
部门表 Department：
+---------------+---------+
| Column Name | Type |
+---------------+---------+
| id | int |
| revenue | int |
| month | varchar |
+---------------+---------+
(id, month) 是表的联合主键。
这个表格有关于每个部门每月收入的信息。
月份（month）可以取下列值 ["Jan","Feb","Mar","Apr","May","Jun","Jul","Aug","Sep","Oct","Nov","Dec"]。
编写一个 SQL 查询来重新格式化表，使得新的表中有一个部门 id 列和一些对应 每个月 的收入（revenue）列。
查询结果格式如下面的示例所示：
Department 表：
+------+---------+-------+
| id | revenue | month |
+------+---------+-------+
| 1 | 8000 | Jan |
| 2 | 9000 | Jan |
| 3 | 10000 | Feb |
| 1 | 7000 | Feb |
| 1 | 6000 | Mar |
+------+---------+-------+
查询得到的结果表：
+------+-------------+-------------+-------------+-----+-------------+
| id | Jan_Revenue | Feb_Revenue | Mar_Revenue | ... | Dec_Revenue |
+------+-------------+-------------+-------------+-----+-------------+
| 1 | 8000 | 7000 | 6000 | ... | null |
| 2 | 9000 | null | null | ... | null |
| 3 | null | 10000 | null | ... | null |
+------+-------------+-------------+-------------+-----+-------------+
注意，结果表有 13 列 (1个部门 id 列 + 12个月份的收入列)。
所有材料及题解来源均出于：力扣（LeetCode）
链接：https://leetcode-cn.com/problems
申明：著作权归领扣网络所有，商业转载请联系官方授权。