What is the SQL for 'next' and 'previous' in a table?

Question

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

Answer 1

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.

Answer 2

Self-joins.

For the table:

/*
CREATE TABLE [dbo].[stackoverflow_203302](
    [val] [int] NOT NULL
) ON [PRIMARY]
*/

With parameter @val

SELECT cur.val, MAX(prv.val) AS prv_val, MIN(nxt.val) AS nxt_val
FROM stackoverflow_203302 AS cur
LEFT JOIN stackoverflow_203302 AS prv
    ON cur.val > prv.val
LEFT JOIN stackoverflow_203302 AS nxt
    ON cur.val < nxt.val
WHERE cur.val = @val
GROUP BY cur.val

You could make this a stored procedure with output parameters or just join this as a correlated subquery to the data you are pulling.

Without the parameter, for your data the result would be:

val         prv_val     nxt_val
----------- ----------- -----------
1           NULL        3
3           1           8
8           3           19
19          8           45
45          19          67
67          45          NULL

For the modified example, you use this as a correlated subquery:

/*
CREATE TABLE [dbo].[stackoverflow_203302](
    [ky] [int] NOT NULL,
    [val] [int] NOT NULL,
    CONSTRAINT [PK_stackoverflow_203302] PRIMARY KEY CLUSTERED (
        [ky] ASC
    )
)
*/

SELECT cur.ky AS cur_ky
        ,cur.val AS cur_val
        ,prv.ky AS prv_ky
        ,prv.val AS prv_val
        ,nxt.ky AS nxt_ky
        ,nxt.val as nxt_val
FROM (
    SELECT cur.ky, MAX(prv.ky) AS prv_ky, MIN(nxt.ky) AS nxt_ky
    FROM stackoverflow_203302 AS cur
    LEFT JOIN stackoverflow_203302 AS prv
        ON cur.ky > prv.ky
    LEFT JOIN stackoverflow_203302 AS nxt
        ON cur.ky < nxt.ky
    GROUP BY cur.ky
) AS ordering
INNER JOIN stackoverflow_203302 as cur
    ON cur.ky = ordering.ky
LEFT JOIN stackoverflow_203302 as prv
    ON prv.ky = ordering.prv_ky
LEFT JOIN stackoverflow_203302 as nxt
    ON nxt.ky = ordering.nxt_ky

With the output as expected:

cur_ky      cur_val     prv_ky      prv_val     nxt_ky      nxt_val
----------- ----------- ----------- ----------- ----------- -----------
1           1           NULL        NULL        2           3
2           3           1           1           3           8
3           8           2           3           4           19
4           19          3           8           5           67
5           67          4           19          6           45
6           45          5           67          NULL        NULL

In SQL Server, I prefer to make the subquery a Common table Expression. This makes the code seem more linear, less nested and easier to follow if there are a lot of nestings (also, less repetition is required on some re-joins).

Answer 3

Try this...

SELECT TOP 3 * FROM YourTable
WHERE Col >= (SELECT MAX(Col) FROM YourTable b WHERE Col < @Parameter)
ORDER BY Col

Answer 4

Select max(element) From Data Where Element < 8

Union

Select min(element) From Data Where Element > 8

But generally it is more usefull to think of sql for set oriented operations rather than iterative operation.

Answer 5

If your RDBMS supports LAG and LEAD, this is straightforward (Oracle, PostgreSQL, SQL Server 2012)

These allow to choose the row either side of any given row in a single query

Answer 6

SELECT 'next' AS direction, MIN(date_field) AS date_key
FROM table_name
  WHERE date_field > current_date
GROUP BY 1 -- necessity for group by varies from DBMS to DBMS in this context
UNION
SELECT 'prev' AS direction, MAX(date_field) AS date_key
  FROM table_name
  WHERE date_field < current_date
GROUP BY 1
ORDER BY 1 DESC;

Produces:

direction  date_key
---------  --------
prev              3
next             19