Question

I have a table of data, and I allow people to add meta data to that table.

I give them an interface that allows them to treat it as though they’re adding extra columns to the table their data is stored in, but I’m actually storing the data in another table.

Data Table    DataID    Data  Meta Table    DataID    MetaName    MetaData 

So if they wanted a table that stored the data, the date, and a name, then I’d have the data in the data table, and the word ‘Date’ in metaname, and the date in MetaData, and another row in the meta table with ‘Name’ in metaname and the name in metadata.

I now need a query that takes the information from these tables and presents it as though coming from a single table with the two additional columns ‘Data’ and ‘Name’ so to the customer it would look like there’s a single table with their custom columns:

MyTable    Data    Date    Name 

Or, in other words, how do I go from here:

Data Table    DataID        Data    1             Testing!    2             Hello, World!  Meta Table    DataID        MetaName         MetaData    1             Date             20081020    1             Name             adavis    2             Date             20081019    2             Name             mdavis 

To here:

MyTable    Data          Date             Name    Testing!      20081020         adavis    Hello, World! 20081019         mdavis 

Years ago when I did this in MySQL using PHP, I did two queries, the first to get the extra meta data, the second to join them all together. I’m hoping that modern databases have alternate methods of dealing with this.

Related to option 3 of this question.

-Adam

Answer 1

You want to pivot each of your name-value pair rows in the MyTable… Try this sql:

DECLARE @Data   TABLE (     DataID      INT IDENTITY(1,1)   PRIMARY KEY,     Data        VARCHAR(MAX) )  DECLARE @Meta   TABLE (     DataID      INT ,     MetaName    VARCHAR(MAX),     MetaData    VARCHAR(MAX) )  INSERT INTO @Data SELECT 'Data'  INSERT INTO @Meta SELECT 1, 'Date', CAST(GetDate() as VARCHAR(20)) UNION SELECT 1, 'Name', 'Joe Test'  SELECT * FROM @Data  SELECT * FROM @Meta  SELECT      D.DataID,     D.Data,     MAX(CASE MetaName WHEN 'Date' THEN MetaData ELSE NULL END) as Date,     MAX(CASE MetaName WHEN 'Name' THEN MetaData ELSE NULL END) as Name FROM     @Meta M JOIN    @Data D     ON M.DataID = D.DataID   GROUP BY     D.DataID,     D.Data