I have a table of items, each of which has a date associated with it. If I have the date associated with one item, how do I query the database with SQL to get the 'previous' and 'subsequent' items in the table?
It is not possible to simply add (or subtract) a value, as the dates do not have a regular gap between them.
One possible application would be 'previous/next' links in a photo album or blog web application, where the underlying data is in a SQL table.
I think there are two possible cases:
Firstly where each date is unique:
Sample data:
1,3,8,19,67,45
What query (or queries) would give 3 and 19 when supplied 8 as the parameter? (or the rows 3,8,19). Note that there are not always three rows to be returned - at the ends of the sequence one would be missing.
Secondly, if there is a separate unique key to order the elements by, what is the query to return the set 'surrounding' a date? The order expected is by date then key.
Sample data:
(key:date) 1:1,2:3,3:8,4:8,5:19,10:19,11:67,15:45,16:8
What query for '8' returns the set:
2:3,3:8,4:8,16:8,5:19
or what query generates the table:
key date prev-key next-key
1 1 null 2
2 3 1 3
3 8 2 4
4 8 3 16
5 19 16 10
10 19 5 11
11 67 10 15
15 45 11 null
16 8 4 5
The table order is not important - just the next-key and prev-key fields.
Both TheSoftwareJedi and Cade Roux have solutions that work for the data sets I posted last night. For the second question, both seem to fail for this dataset:
(key:date) 1:1,2:3,3:8,4:8,5:19,10:19,11:67,15:45,16:8
The order expected is by date then key, so one expected result might be:
2:3,3:8,4:8,16:8,5:19
and another:
key date prev-key next-key
1 1 null 2
2 3 1 3
3 8 2 4
4 8 3 16
5 19 16 10
10 19 5 11
11 67 10 15
15 45 11 null
16 8 4 5
The table order is not important - just the next-key and prev-key fields.
My own attempt at the set solution, based on TheSoftwareJedi.
First question:
select date from test where date = 8
union all
select max(date) from test where date < 8
union all
select min(date) from test where date > 8
order by date;
Second question:
While debugging this, I used the data set:
(key:date) 1:1,2:3,3:8,4:8,5:19,10:19,11:67,15:45,16:8,17:3,18:1
to give this result:
select * from test2 where date = 8
union all
select * from (select * from test2
where date = (select max(date) from test2
where date < 8))
where key = (select max(key) from test2
where date = (select max(date) from test2
where date < 8))
union all
select * from (select * from test2
where date = (select min(date) from test2
where date > 8))
where key = (select min(key) from test2
where date = (select min(date) from test2
where date > 8))
order by date,key;
In both cases the final order by clause is strictly speaking optional.