目录
方法二：row_number()
1、180连续出现的数字
编写一个 SQL 查询，查找所有至少连续出现三次的数字。返回的结果表中的数据可以按任意顺序排列。
Create table If Not Exists Logs (id int, num int);
Truncate table Logs;
insert into Logs (id, num) values ('1', '1');
insert into Logs (id, num) values ('2', '1');
insert into Logs (id, num) values ('3', '1');
insert into Logs (id, num) values ('4', '2');
insert into Logs (id, num) values ('5', '1');
insert into Logs (id, num) values ('6', '2');
insert into Logs (id, num) values ('7', '2');
SELECT * FROM LOGS;
方法一：自连接查询
由于数据比较少，可以考虑自连接的方式
查找num数据相同，且id差距为1,可用于确定连续次数，且次数较少。
SELECT e1.id, e1.num AS ConsecutiveNums
FROM Logs e1,LOGS e2,LOGS e3
WHERE e1.num=e2.num
AND e2.num=e3.num
AND e1.id-e2.id=1
AND e2.id-e3.id=1;方法二：row_number()
用于数据较多，连续次数较多
select distinct Num as ConsecutiveNums from(
select Num,(row_number()over(order by Id)-row_number()over(partition by Num order by Id)) as series_id
from Logs) t
group by Num, series_id
having count(1) >= 3;
扩展
row_number()函数的用法：
语法：ROW_NUMBER() OVER(PARTITION BY COLUMN ORDER BY COLUMN)
row_number() OVER (PARTITION BY e1 ORDER BY e2) 表示根据e1分组，在分组内部根据 e2排序，而此函数计算的值就表示每组内部排序后的顺序编号（组内连续的唯一的)
2、181超过经理收入的员工
编写一个SQL查询来查找收入比经理高的员工。以任意顺序返回结果表。
Create table If Not Exists Employee1 (id int, name varchar(255), salary int, managerId int);
Truncate table Employee1;
insert into Employee1 (id, name, salary, managerId) values ('1', 'Joe', '70000', '3');
insert into Employee1 (id, name, salary, managerId) values ('2', 'Henry', '80000', '4');
insert into Employee1 (id, name, salary, managerId) values ('3', 'Sam', '60000', NULL);
insert into Employee1 (id, name, salary, managerId) values ('4', 'Max', '90000', NULL);
SELECT * FROM employee1;
方法一：where语句
使用where语句，自连接查询，从两个表中使用select语句会产生笛卡尔乘积，用where语句筛选行。
SELECT e1.name as employee
FROM employee1 e1,employee e2
WHERE e1.managerId=e2.id
and e1.salary>e2.salary方法二：join语句
使用join语句，连接同一个表，用第一个表managerId和第二个表的id来连接
SELECT e1.name as employee
FROM employee1 as e1
JOIN employee1 as e2
ON e1.managerId=e2.id
and e1.salary>e2.salary;
3、182查找重复的电子邮箱
编写一个 SQL 查询，查找 Person 表中所有重复的电子邮箱。
Create table If Not Exists Person1 (id int, email varchar(255));
Truncate table Person1;
insert into Person1 (id, email) values ('1', '[email protected]');
insert into Person1 (id, email) values ('2', '[email protected]');
insert into Person1 (id, email) values ('3', '[email protected]');
SELECT * FROM Person1;
与上题解题思路一致
方法一：where语句
方法二：join语句
方法三：使用group by和临时表
方法四：调用group by和having语句
#方法一：
SELECT distinct e1.email
FROM person1 e1,person1 e2
WHERE e1.email=e2.email
AND e1.id>e2.id;
#方法二：
SELECT distinct e1.email FROM person1 as e1
join person1 as e2
ON e1.email=e2.email
AND e1.id>e2.id;
#方法三
select distinct Email
from (select email,count(email) as num from person1 group by email) as e1
where num>1;
方法四：
select Email from person1 g
roup by Email
having count(email)>1;
4、183从不订购的客户
某网站包含两个表，Customers 表和 Orders 表。编写一个 SQL 查询，找出所有从不订购任何东西的客户。
Create table If Not Exists Customers (id int, name varchar(255));
Create table If Not Exists Orders (id int, customerId int);
Truncate table Customers;
insert into Customers (id, name) values ('1', 'Joe');
insert into Customers (id, name) values ('2', 'Henry');
insert into Customers (id, name) values ('3', 'Sam');
insert into Customers (id, name) values ('4', 'Max');
Truncate table Orders;
insert into Orders (id, customerId) values ('1', '3');
insert into Orders (id, customerId) values ('2', '1');
SELECT * FROM Customers;
SELECT * FROM Orders;
观察两个数据表可以发现，Customers.id与Orders.customerId关联，可通过子查询查到订购过的客户名单，用not in 子句排除订购过的客户名单，得到从不订购的客户名单。
select name as 'Customers' from Customers
where Customers.id
not in(select CustomerId from Orders)
错误案例：通过子查询查询购买客户的名字，用not in子句排除订购过的客户名单，得到从不订购的客户名单。
错误原因：客户id是唯一的，客户名字可能重名。
select name as Customers from Customers
where name
NOT in (select name from Customers join Orders
on Customers.id=Orders.CustomerId)转载说明：题目出自LeetCode官网