Leetcode Day2 consecutive numbers

180. Consecutive numbers

Medium difficulty

SQL framework

surface ：Logs

+-------------+---------+
| Column Name | Type    |
+-------------+---------+
| id          | int     |
| num         | varchar |
+-------------+---------+
id  It's the primary key of this table .

Write a SQL Inquire about , Find all numbers that appear at least three times in a row .

The data in the returned result table can be displayed by In any order array .

The query result format is shown in the following example ：

Logs  surface ：
+----+-----+
| Id | Num |
+----+-----+
| 1  | 1   |
| 2  | 1   |
| 3  | 1   |
| 4  | 2   |
| 5  | 1   |
| 6  | 2   |
| 7  | 2   |
+----+-----+

Result  surface ：
+-----------------+
| ConsecutiveNums |
+-----------------+
| 1               |
+-----------------+
1  Is the only number that appears at least three times in a row .

Solution 1 ：

​ Find the current through self table connection id Connect the next one of with the next one , Then get the current through condition judgment num Equal to the following two num Of num value , Then perform the de duplication operation , In this way, three consecutive occurrences of num, This way of writing does not consider id Discontinuous , And if n Sub continuous situation , Low availability

Code example ：

select distinct a.Num as ConsecutiveNums from logs as a 
left join logs as b on a.id = b.id+1
left join logs as c on a.id = c.id+2
where a.Num = b.Num and a.Num = c.Num;

Solution 2 ：

1、 First consider if id In case of discontinuity, we need to query one more column to construct a continuous column , Here you can use the window function row_number()

select *,row_number() over (order by id) as new_id from Logs;

In this way, if id Discontinuous , Build a continuous column by querying new_id, The specific implementation is shown in the following table

id	num	new_id
1	2	1
3	2	2
4	3	3
7	3	4
8	3	5
9	2	6

2、 Then through grouping query num Consistent column grouping Sorting Query

select *,row_number over (partition by Num  order by id) as new_order from Logs;

The operation results are shown in the following table

id	num	new_order
1	2	1
3	2	2
9	2	3
4	3	1
7	3	2
8	3	3

sql Example ：

select *,row_number() over(order by id) as new_id,
row_number() over (partition by Num order by id) as new_order
from Logs;

The operation results are shown in the following table ：

id	num	new_id	new_order
1	2	1	1
3	2	2	2
9	2	6	3
4	3	3	1
7	3	4	2
8	3	5	3

Combining these two queries, we can find the rule ：

use new_id subtract new_order As long as the value of is the same , Then they are continuous . That is to say ： If one num Continuous occurrence , Then it appears [ Real sequence ]- The number of times it appears must be a fixed value . Their thinking ： The first step is to get the real sequence of his appearance , The second step is to get the number of times it appears , Then the next step is to subtract them , If the value is the same after subtraction , Then they must be continuous .

3、[ Real sequence ]- The number of times it appears

select *,
row_number() over(order by id) - row_number() over (partition by Num order by id) as sta,
from Logs;

Operation result table ：

id	num	sta
1	2	0
3	2	0
9	2	3
4	3	2
7	3	2
8	3	2

It can be seen that there are two consecutive numbers , One is id by 1、3 Two consecutive , The other is 4、7、8 Three in a row , According to the data, there is no problem , Then we can find out the continuous occurrence through this n Secondary data .

select distinct Num as ConsecutiveNums 
from
(
    select id,Num,
    row_number() over (order by id) - row_number() over (partition by Num order by id) as sta from Logs
) as Sub group by Num , sta having count(1)>=3

ad locum Inquire about The result set of subquery , adopt Num grouping , If the number of consecutive occurrences exceeds three, it will be changed Num Find out , And carry out the de duplication operation , To query n The secondary data only needs to be having In the condition of count(1) The condition is changed to be greater than n that will do .

Running results ：

ConsecutiveNums : 1

summary ：

The first solution is easy to understand and realize , But there are some limitations .

The second solution is to solve problems through laws , The implementation is relatively complex , But it's more practical .