Question

I am having trouble figuring out how to even start this query.

I have a table that has the following columns and data:

User   BeginMile EndMile
1         1        5
1         5        6
1         6        20
1         20       25
1         25       29
2         1         9
2         15        20
3         1         2
3         6         10
3         10        12

I need to first find where there are gaps for each user from the EndMile of the previous record, to the BeginMile of the next record. I then need to return the record before and after where the gap occurs for each user.

In the previous data example, I would like the following returned:

User   PrevBeginMile   PrevEndMile    AfterBeginMile AfterEndMile   Gap
2         1                9                15          20           6
3         1                2                6           10           4

How can this be done?

Answer 1

Considering you’re on SQL 2005, this should work:

DECLARE @Runners TABLE (Id INT, BeginMile INT, EndMile INT)

INSERT INTO @Runners VALUES (1,1,5)
INSERT INTO @Runners VALUES (1,5,6)
INSERT INTO @Runners VALUES (1,6,20)
INSERT INTO @Runners VALUES (1,20,25)
INSERT INTO @Runners VALUES (1,25,29)
INSERT INTO @Runners VALUES (2,1,9)
INSERT INTO @Runners VALUES (2,15,20)
INSERT INTO @Runners VALUES (3,1,2)
INSERT INTO @Runners VALUES (3,6,10)
INSERT INTO @Runners VALUES (3,10,12)

WITH OrderedUsers AS (
    SELECT  *
    ,   ROW_NUMBER() OVER (PARTITION BY Id ORDER BY BeginMile) RowNum
    FROM    @Runners
)

SELECT  a.Id [User]
,   a.BeginMile PrevBeginMile
,   a.EndMile PrevEndMile
,   b.BeginMile AfterBeginMile
,   b.EndMile AfterEndMile
,   b.BeginMile - a.EndMile Gap
FROM    OrderedUsers a
JOIN    OrderedUsers b
ON  a.Id = b.Id
AND a.EndMile <> b.BeginMile
AND a.RowNum = b.RowNum - 1