Question

I have a requirement that I’m struggling to implement. If possible, I’d like to achieve this with native T-SQL.

I have the following tables:

CUSTOMER
========
ID,
Name

FIELDDEF
========
ID,
Name
FieldType   (Char  T, N, D   for Text, Number or Date)

CUSTOMERFIELD
=============
ID,
CustomerID,
FieldDefID,
CaptureDate,
ValueText,
ValueNumber,
ValueDate

Basically, the purpose of these tables is to provide an extensible custom field system. The idea is that the user creates new field definitions that can be a text, number or date field. Then, they create values for these fields in the ValueText, ValueNumber OR ValueDate field.

Example:

*Customer*
1,BOB
2,JIM

*FieldDef*
1,Mobile,T
1,DateOfBirth,D

*CustomerField*
ID,CustomerID,FieldDefID,CaptureDate,ValueText,ValueNumber,ValueDate
1,1,1,2011-01-1,07123456789,NULL,NULL
2,1,2,2011-01-1,NULL,NULL,09-DEC-1980
3,1,1,2011-01-2,07123498787,NULL,NULL

I need to create a view that looks like this:

*CustomerView*
ID,Name,Mobile,DateOfBirth
1,BOB,07123498787,09-DEC-1980

Note that Bob’s mobile is the second one in the list, because it uses the most recent capture date.

Ideally, I need this to be extensible, so if I create a new field def in the future, it is automatically picked up in the CustomerView.

Is this possible in T-SQL at all?

Thanks,

Simon.

Answer 1

This would not be possible with a view, unless the view is dynamically recreated on the fly every time FieldDef changes because view schemas are locked-in at creation time. However, it may be possible with a stored procedure, which may or may not work depending on how you are using it.

Edit 1

Here is a sample query that works just for your current field names, and would have to be modified by dynamic SQL to work in general:

Edit 2

Modified to grab the newest values from the customer field table

with CustomerFieldNewest as (
    select
        cf1.*
    from
        customerfield cf1
    inner join
        (
            select
                customerid,
                fielddefid,
                max(capturedate) as maxcapturedate
            from
                customerfield cf2
            group by
                customerid,
                fielddefid
        ) cf2 on cf1.customerid = cf2.customerid
            and cf1.fielddefid = cf2.fielddefid
            and cf1.capturedate = cf2.maxcapturedate
)
,CustomerFieldPivot as (
    select
        C.ID as ID
        ,max(case when F.Name = 'Mobile' then CF.ValueText end) as Mobile
        ,max(case when F.Name = 'DateOfBirth' then CF.ValueDate end) as DateOfBirth
    from
        Customer C
    left join
        CustomerFieldNewest CF on C.ID = CF.CustomerID
    left join
        FieldDef F on F.ID = CF.FieldDefID
    group by
        C.ID
)
select
    C.*
    ,P.Mobile
    ,P.DateOfBirth
from
    Customer C
left join
    CustomerFieldPivot P on C.ID = P.ID

Edit 3

Here is T-SQL code to generate the view on the fly based on the current set of fields in FieldDef (this assumes the view CustomerView already exists, so you will need to create it first as a blank definition or you will get an error). I’m not sure about the performance of all this, but it should work correctly.

declare @sql varchar(max)
declare @fielddef varchar(max)
declare @fieldlist varchar(max)

select
    @fielddef = coalesce(@fielddef + ', ' + CHAR(13) + CHAR(10), '') +
        '           max(case when F.Name = ''' + F.Name + ''' then CF.' +
            case F.FieldType
                when 'T' then 'ValueText'
                when 'N' then 'ValueNumber'
                when 'D' then 'ValueDate'
            end
        + ' end) as [' + F.Name + ']'

    ,@fieldlist = coalesce(@fieldlist + ', ' + CHAR(13) + CHAR(10), '') +
        '       [' + F.Name + ']'
from
    FieldDef F

set @sql = '
    alter view [CustomerView] as

    with CustomerFieldNewest as (
        select
            cf1.*
        from
            customerfield cf1
        inner join
            (
                select
                    customerid,
                    fielddefid,
                    max(capturedate) as maxcapturedate
                from
                    customerfield cf2
                group by
                    customerid,
                    fielddefid
            ) cf2 on cf1.customerid = cf2.customerid
                and cf1.fielddefid = cf2.fielddefid
                and cf1.capturedate = cf2.maxcapturedate
    )
    ,CustomerFieldPivot as (
        select
            C.ID as ID,
' + @fielddef + '
        from
            Customer C
        left join
            CustomerFieldNewest CF on C.ID = CF.CustomerID
        left join
            FieldDef F on F.ID = CF.FieldDefID
        group by
            C.ID
    )
    select
        C.*,
' + @fieldlist + '
    from
        Customer C
    left join
        CustomerFieldPivot P on C.ID = P.ID
'

print @sql
exec(@sql)

select * from CustomerView