Hi pierjsap,
I too am somewhat confused. It sounds like you're asking if the SQL data source can read data? The answer is yes, it can store data from the data source's select statement or it can be configured only to update the table (which it must be if you wish to set the point to some dynamic value in the update statement, a point that isn't "Modify Table Only" will only have a constant update statement), You either have to match the 'Column name' property of the point to the column names returned, or do a "row based query" and have the column name match the value in the first column of the results, then its value in the second column.
There are a few examples scattered around the forum. If you can say more, perhaps I can recall a relevant thread for you.