select avg(if(datediff(date2, date1)=1, 1, 0)) as avg_ret from ( select distinct device_id, date as date1, lead(date) over (partition by device_id order by date) as date2 from ( select distinct device_id, date from question_practice_detail ) as uniq_id_date ) as id_last_next_date
时间: 2024-04-14 09:31:57 浏览: 116
您的第一个问题是:对于给定的查询,它是用来计算每个设备的连续日期之间的平均间隔的吗?
是的,您理解得很正确!这个查询用于计算每个设备的连续日期之间的平均间隔。首先,内部查询 `uniq_id_date` 在 `question_practice_detail` 表中选择了独立的设备ID和日期。然后,外部查询使用 `lead()` 函数为每个设备的日期计算下一个日期。最后,外部查询使用 `datediff()` 函数计算相邻日期之间的间隔,并将间隔为1的日期计数为1,其他日期计数为0。然后,通过对每个设备的计数求平均值,得到了所有设备的连续日期之间的平均间隔。查询结果将作为别名 `avg_ret` 返回。
请注意,这是根据您提供的查询进行的解释,如果有任何遗漏或误解,请随时纠正。
相关问题
create trigger tri_jieshu on reader_borrow for insert as declare @id varchar(50) select @id=borrow_id from inserted update book_borrowing set book_surplus=book_surplus-1 where book_id=@id create trigger tri_huanshu on reader_back for insert as declare @id varchar(50) select @id=back_id from inserted update book_borrowing set book_surplus=book_surplus+1 where book_id=@id create proc payment @id varchar(20),@name varchar(20) as begin select borrow_date,back_date from reader_back,reader_borrow where back_id=borrow_id and back_id=@id and borrow_bookname=back_bookname and (select datediff(day, borrow_date,back_date) FROM reader_back,reader_borrow)>7 end create view view_pay as select * from reader_back,reader_borrow where (borrow_id=back_id and borrow_bookname=back_bookname) create proc proc_pay (@id varchar(20)) as select back_id,back_bookname,back_date,borrow_date from view_pay where back_id=@id exec proc_pay @id='1'该代码有什么语法错误
在触发器 tri_jieshu 和 tri_huanshu 中,update 语句中的 where 子句需要指定 book_id 而不是 borrow_id 和 back_id。应该修改为:
```
update book_borrowing set book_surplus=book_surplus-1 where book_id=(select book_id from inserted)
```
和
```
update book_borrowing set book_surplus=book_surplus+1 where book_id=(select book_id from inserted)
```
在 payment 存储过程中,应该将 datediff 函数的返回值赋值给一个变量,然后使用该变量进行比较。应该修改为:
```
create proc payment @id varchar(20),@name varchar(20) as
begin
declare @days int
select @days = datediff(day, borrow_date,back_date) FROM reader_back,reader_borrow
where back_id=borrow_id and back_id=@id and borrow_bookname=back_bookname
if (@days > 7)
begin
select borrow_date,back_date from reader_back,reader_borrow
where back_id=borrow_id and back_id=@id and borrow_bookname=back_bookname
end
end
```
在 proc_pay 存储过程中,应该将 @id 的数据类型修改为 varchar(50),并且在查询结果中使用别名来避免列名冲突。应该修改为:
```
create proc proc_pay (@id varchar(50)) as
begin
select b.back_id, b.back_bookname, b.back_date, r.borrow_date
from reader_back b, reader_borrow r
where b.back_id = r.borrow_id and b.back_id = @id and b.borrow_bookname = r.back_bookname
end
exec proc_pay @id='1'
```
mysql> create procedure pro_working_years() -> begin -> delare @hire_date date; -> select @hire_date= hire_date from emp where emp_id = @emp_id; -> set @working_years = datediff(year,@hire_date,getdate()); -> end -> // ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '@hire_date date; select @hire_date= hire_date from emp where emp_id = @emp_id; s' at line 3
在这个存储过程中,第三行有一个语法错误。正确的语法应该是DECLARE而不是DELARE。因此,正确的存储过程代码如下:
```
mysql> CREATE PROCEDURE pro_working_years()
-> BEGIN
-> DECLARE @hire_date DATE;
-> SELECT @hire_date = hire_date FROM emp WHERE emp_id = @emp_id;
-> SET @working_years = DATEDIFF(YEAR, @hire_date, GETDATE());
-> END
-> //
```
请注意,这只是一个基本的存储过程示例,需要根据您的实际需求进行修改。
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