SQL cross apply

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