How to get the MAX(Hour) and The MIN(Hour) for eac

问题:

I have this structure of table Diary:

CREATE TABLE Diary
(
     [IdDiary] bigint, 
     [IdDay] numeric(18,0)
);

INSERT INTO Diary ([IdDiary], [IdDay])
values 
(51, 1),
(52, 2),
(53, 5);

And this other structure for table DiaryTimetable:

  CREATE TABLE DiaryTimetable
(
     [IdDiary] bigint, 
     [Hour] varchar(50)
);

  INSERT INTO DiaryTimetable ([IdDiary], [Hour])
VALUES
    (51, '09:00'),
    (51, '09:30'),
    (51, '10:00'),
    (51, '10:30'),
    (51, '11:00'),
    (51, '11:30'),
    (52, '11:00'),
    (52, '11:30'),
    (52, '12:00'),
    (52, '12:30'),
    (52, '13:00'),
    (52, '13:30'),
    (53, '15:00'),
    (53, '15:30'),
    (53, '16:00'),
    (53, '16:30');

The table Diary contains an IdDiary and the IdDay is the number of day, for example:

Monday --> 1
Tuesday --> 2
Wednesday --> 3
Thursday --> 4
Friday --> 5
Saturday --> 6
Sunday --> 7

The table DiaryTimetable contains the iddiary, and the hour. I want want to get the max hour and the min hour in the table DiaryTimetable for each day appears in the Diary table, If I put this query the result will be only the max hour and the min hour for all the query:

select MAX(Hour), MIN(Hour) from DiaryTimetable
inner join Diary on
DiaryTimetable.IdDiary = Diary.IdDiary

The result for wat I need will be something like that:

IdDiary  IdDay   Min Hour   Max Hour
-----    -----   --------   ---------
51        1      09:00       11:30
52        2      11:00       13:30
53        5      15:00       16:30

How can I get this, thanks?

SQL FIDDLE DEMO HERE

回答1:

You use a GROUP BY clause:

SELECT d.IdDiary, d.IdDay, MIN(Hour), MAX(Hour)
FROM Diary AS d
LEFT JOIN DiaryTimetable AS dt ON d.IdDiary = dt.IdDiary
GROUP BY d.IdDiary, d.IdDay

Demo here

回答2:

You just missed Group by in your query.

   SELECT b.IdDiary,a.IdDay, MIN(b.Hour), MAX(b.hour)
   from  Diary A INNER JOIN   DiaryTimetable B  ON A.IdDiary=B.IdDiary  
   GROUP by  B.IdDiary   ,a.IdDay 

回答3:

You just need to add a GROUP BY to your query:

SELECT Diary.IdDiary, Diary.IdDay, MAX(DT.Hour), MIN(DT.Hour) 
FROM DiaryTimetable AS DT
INNER JOIN Diary ON
DT.IdDiary = Diary.IdDiary
GROUP BY Diary.IdDiary, Diary.IdDay

Then the MAXand MINcommand just use the Values from your Diary Table.

回答4:

As in a comment you say that this is just an example for you to learn, I'd like to add another answer.

Given that there are no duplicate records in Diary, its two columns IdDiary and IdDay are the fields you want additional information on (the min and max hour). So all you have to do is get this information joined to the table:

select
  d.iddiary,
  d.idday,
  dt.min_hour,
  dt.max_hour
from diary d
join
(
  select
    iddiary,
    min(hour) as min_hour,
    max(hour) as max_hour
  from diarytimetable
  group by iddiary
) dt on dt.iddiary = d.iddiary;

Sticking to this technique has two advantages:

1. You show very clearly in your select what you are doing and how the tables are related.
2. This works with several aggregates from different tables. Some people, especially beginners, tend to join all tables first and only then think about how to get to the data needed. This can be quite troublesome. Say you have a table city, a table park and a table lake. You want to know how many lakes and the sum of park area per city. As shown you'd join the cities with the lake count and the park area sum. If you joined all tables first instead, you'd have a combination of lakes and parks that are actually really unrelated, and it would be hard or impossible to segregate the desired results from this data slush somehow.