SQL cross apply - sql

Question

I have a SQL table which contains audit information:
GroupId AuditDate ID FirstName LastName
1 01/06/2011 123 Michael Jackson
1 01/09/2010 123 M J
1 01/06/2009 123 Mike J
and trying show the differences between the audit records:
GroupId AuditDate ID Attribute From To
1 01/06/2011 123 FirstName M Michael
1 01/06/2011 123 LastName J Jackson
1 01/09/2010 123 FirstName Mike M
1 01/06/2009 123 FirstName NULL Mike
1 01/06/2009 123 LastName NULL J
I am using the following SQL query:
WITH result AS (
SELECT [Current].Id,
[Current].GroupId,
[Current].AuditDate,
[Current].FirstName,
[Current].LastName
Previous.FirstName AS PFirstName,
Previous.LastName AS PLastName,
FROM
(SELECT
*, ROW_NUMBER() OVER(PARTITION BY GroupId ORDER BY AuditDate ASC) AS RowNumber
FROM
AuditTable
WHERE
Id = #ID
) AS [Current]
LEFT JOIN
(SELECT
*, ROW_NUMBER() OVER(PARTITION BY GroupId ORDER BY AuditDate ASC) AS RowNumber
FROM
AuditTable
WHERE
Id = #ID
) AS [Previous]
ON
[Current].RowNumber = [Previous].RowNumber + 1
)
SELECT r.Id,r.GroupId, r.AuditDate
x.Attribute,
x.[From],
x.[To]
FROM result r
CROSS APPLY
(
VALUES
('FirstName', t.FirstName, t.PFirstName),
('LastName', t.LastName, t.PLastName),
) x (Attribute, [To], [From])
where
ISNULL(x.[From],'') <> ISNULL(x.[To],'')
ORDER BY r.AuditDate asc;
Is it possible to merge two select queries to improve the performance?

Answer 1

Try this query
WITH result AS (
SELECT Id,
GroupId,
AuditDate,
FirstName,
LastName,
ROW_NUMBER() OVER(PARTITION BY GroupId ORDER BY AuditDate ASC) AS RowNumber
FROM AuditTable
WHERE Id = #ID
)
SELECT r.Id,r.GroupId, r.AuditDate,
x.Attribute,
x.[From],
x.[To]
FROM result r LEFT JOIN result r2 ON r.RowNumber = r2.RowNumber + 1
CROSS APPLY (
VALUES ('FirstName', r.FirstName, r2.FirstName),
('LastName', r.LastName, r2.LastName)
) x (Attribute, [To], [From])
WHERE ISNULL(x.[From],'') <> ISNULL(x.[To],'')
ORDER BY r.AuditDate ASC;
Demo on SQLFiddle

Answer 2

You can eliminate both subqueries entirely by using lag():
WITH result AS (
SELECT Id,
GroupId,
AuditDate,
FirstName,
LastName,
lag(FirstName) over (PARTITION BY GroupId ORDER BY AuditDate ASC)
AS PFirstName,
lag(LastName) over (PARTITION BY GroupId ORDER BY AuditDate ASC)
AS PLastName
FROM AuditTable
WHERE Id = #ID
)
...
Here is the relevant documentation.
Update: However, this is only available in SQL Server 2012, unfortunately. If you have an earlier version, you will need some sort of self join.
If you can't use lag(), you should at least be able to reduce your code from 3 queries to 2: include the row number in your first select statement, and left join one subquery to it rather than having two subqueries. I'm not sure whether this way or Chris Moutray's way would be faster.
WITH result AS (
SELECT ROW_NUMBER() OVER(PARTITION BY GroupId ORDER BY AuditDate ASC) AS RowNumber
[Current].Id,
[Current].GroupId,
[Current].AuditDate,
[Current].FirstName,
[Current].LastName
[Previous].FirstName AS PFirstName,
[Previous].LastName AS PLastName,
FROM AuditTable as [Current]
LEFT JOIN
(SELECT
*, ROW_NUMBER() OVER(PARTITION BY GroupId ORDER BY AuditDate ASC) AS RowNumber
FROM
AuditTable
WHERE
Id = #ID
) AS [Previous]
ON
[Current].RowNumber = [Previous].RowNumber + 1
)

Answer 3

You can use LAG in SQL Server 2012. I've used UNION ALL here to unpivot columns into rows.
Depending on how you filter and what your group level is, add/modify the PARTITION BY
DECLARE #foo TABLE (GroupId tinyint, AuditDate date, ID tinyint, FirstName varchar(100), LastName varchar(100));
INSERT #foo VALUES (1, '20110601', 123, 'Michael', 'Jackson'), (1, '20100901', 123, 'M', 'J'), (1, '20090601', 123, 'Mike', 'J');
SELECT
X.GroupId, X.AuditDate, X.ID, X.[From], X.[To]
FROM
(
SELECT
F.GroupId, F.AuditDate, F.ID, 'FirstName' AS Attribute, LAG(F.FirstName) OVER (/*PARTITION BY GroupId, ID*/ ORDER BY AuditDate) AS [From], F.FirstName AS [To]
FROM
#foo F
UNION ALL
SELECT
F.GroupId, F.AuditDate, F.ID, 'LastName' AS Attribute, LAG(F.LastName) OVER (/*PARTITION BY GroupId, ID*/ ORDER BY AuditDate) AS [From], F.LastName AS [To]
FROM
#foo F
) X
WHERE
ISNULL(X.[From], '') <> ISNULL(X.[To], '')
ORDER BY
X.AuditDate DESC, X.Attribute