Fun with SQL: Joining on column name and row field value

Ok, it is that time of the day where I’m starting to get high on caffeine 🙂

So this is the scenario. I have an application that is automatically generating a table in my database. It can add columns on the fly(But thankfully not delete).

It could look like this:

CREATE TABLE Cube.AppGenereatedTable
(
x1value float,
x2value float,
ID_Region INT
)

We can now query information about that table like so:

select column_name,ordinal_position,DATA_TYPE , *  from information_schema.columns
where table_name = 'AppGenereatedTable'
AND TABLE_SCHEMA='cube'

The problem is, that right now, I do not know what to do with that data, and when I’m not at the customer anymore, more fields can be added by the application. What I would like, is a place where the customer can add more information about that column, and decide if that column should be included in some logic. To do that, I add a “metadata” table.

CREATE TABLE Dim.SomeMetadata
(
ID_SomeMetadata INT,
ColName nvarchar(100),
friendlyName nvarchar(100),
includeInETLLogig bit
)

Now I want to put it all in a fact table, where I have 1 row / column value. It could look like this:

CREATE TABLE Fact.SomeFactTable
(
Value float,
ID_Metadata INT
)

Ehhh, Ok, so know I will do what the title is saying. To load the data, I need to create the sql automaticly like so:

--Use the colnr to reference all collumns
DECLARE @colnr AS INT
DECLARE @sql AS NVARCHAR(MAX)
SET @colnr = 1
--Do for as many collumns you expect there ever will be. Do some automation here if you like
WHILE (@colnr < 50)
BEGIN
SET  @sql =
'INSERT INTO Fact.SomeFactTable
(Value, ID_Metadata)

--And now we do the SQL with the join. Pay attention 

SELECT '+(select column_name from information_schema.columns
where table_name = 'AppGenereatedTable'
AND TABLE_SCHEMA='cube'
AND ordinal_position=@colnr AND DATA_TYPE = 'float')+' as value
,someMetaData.ID_SomeMetadata
FROM Cube.AppGenereatedTable appGenereatedTable
INNER JOIN Dim.SomeMetadata someMetaData on someMetaData.ColName='''+(select
column_name from information_schema.columns
where table_name = 'AppGenereatedTable'
AND TABLE_SCHEMA='cube'
AND ordinal_position=@colnr AND DATA_TYPE = 'float')+''''

print @colnr
print @sql
BEGIN TRY
exec( @sql )
END TRY
BEGIN CATCH
--Catch error. Do nothing
END CATCH
SET @colnr = @colnr + 1
END

Well, I don’t know if there is other than me, who can use this for something. Next post will be about precisely about what I’m using this to 🙂

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