After determined the sheets and the fields in the database, it is needed to establish the associations between the data sheets, to facilitate the multi-sheet query. According to corresponding associations recorded between the data sheets, the data sheet associations can be divided into three kinds, namely oneto-one associations, one-to-many associations and many-to-many associations. Regarding to many-tomany associations it can use the third table to its transform itself to two one-to-one associations.