将MSSQL转换为MySQL

Question

是的，我正在尝试将MSSQL查询转换为MYSQL，这导致了我的问题。这超出了我目前的舒适区。以下是我当前的查询。

WITH n AS ( 
  SELECT n from (values(0),(1),(2),(3),(4),(5),(6),(7),(8),(9)) t(n) /* create a numbers table with 10 rows */ 
) 
, d AS ( /* Create a table with a row for each day in the date range */ 
         /* Use cross join to increase the rows in this table and then use top() to only return the rows we need */ 
   SELECT top (datediff(day, '2017-07-04', '2018-03-02')+1) 
   SessionDate = convert(datetime,dateadd(day,row_number() over(order by (select 1))-1,'2017-07-04')) 
   FROM n AS ten 
   CROSS JOIN n AS hundred /* cross join the numbers table to create 100 rows */ 
   CROSS JOIN n AS thousand /* cross join the numbers table to create 1,000 rows */ 
   CROSS JOIN n AS tenK /* cross join the numbers table to create 10,000 rows */ 
   CROSS JOIN n AS hundredK /* cross join the numbers table to create 100,000 rows */ 
   ORDER BY SessionDate 
) 
, h as ( /* add time ranges to date table */ 
    SELECT SessionDate, StartDateTime = dateadd(hour,v.s,SessionDate), EndDateTime = dateadd(hour,v.e,SessionDate), v.point 
    FROM d 
    CROSS APPLY (values 
       (0,12,'morning') 
       ,(12,17,'afternoon') 
       ,(17,24,'evening') 
    ) 
    v (s,e,point) 
) 
SELECT *
FROM h

它使用数字表并将日期分成不同的时间范围。以下是结果集的示例

SessionDate             | StartDateTime           | EndDateTime             | Point
2017-07-04 00:00:00.000 | 2017-07-04 00:00:00.000 | 2017-07-04 12:00:00.000 | morning
2017-07-04 00:00:00.000 | 2017-07-04 12:00:00.000 | 2017-07-04 17:00:00.000 | afternoon
2017-07-04 00:00:00.000 | 2017-07-04 17:00:00.000 | 2017-07-05 00:00:00.000 | evening
2017-07-05 00:00:00.000 | 2017-07-05 00:00:00.000 | 2017-07-05 12:00:00.000 | morning
2017-07-05 00:00:00.000 | 2017-07-05 12:00:00.000 | 2017-07-05 17:00:00.000 | afternoon
2017-07-05 00:00:00.000 | 2017-07-05 17:00:00.000 | 2017-07-06 00:00:00.000 | evening
2017-07-06 00:00:00.000 | 2017-07-06 00:00:00.000 | 2017-07-06 12:00:00.000 | morning
2017-07-06 00:00:00.000 | 2017-07-06 12:00:00.000 | 2017-07-06 17:00:00.000 | afternoon
2017-07-06 00:00:00.000 | 2017-07-06 17:00:00.000 | 2017-07-07 00:00:00.000 | evening

Answer 1

查询太聪明了 - 它使用CTE生成一系列数字而不是日历表，它使用CROSS APPLY生成小时而不是使用查找表表示小时和名称。由此产生的执行计划将是糟糕的。

一个非常简单的日历表和一个'Sessions`表将允许您创建一个更简单的T-SQL查询，例如：

CREATE TABLE Calendar (Date date primary key not null)
GO
--Omit code to fill the calendar
CREATE TABLE Sessions (StartTime int,EndTime int, Name nvarchar(20))
GO
insert into Sessions (StartTime,EndTime,Name)
VALUES
(0,12,'morning'),
(12,17,'afternoon'),
(17,24,'evening') 


select Date as SessionDate,
    dateadd(hour,StartTime,Date) as StartTime,
    dateadd(hour,EndTime,Date) as EndTimeTime,
    Name
from Calendar,Sessions
where Date between @startDate and @endDate

范围查询将非常快，因为Date列已编制索引。执行计划将简单地返回所有适用的日期行，并将它们与会话行组合。

这可以很容易地转换为MySQL的方言，例如使用DATE_ADD而不是DATEADD