SQL Join Field and Content
According with your last edit:

SELECT
    F.*,
    C.*
FROM db1.Fields AS F
JOIN db1.Content AS C ON (C.ID = F.ID AND C.ForUser = F.ForUser 
                          AND C.ForCategory = F.ForCategory)
WHERE `the field you want to filter`=`the value you want to search`;


You can filter with Any field in Fields or Content.