When a data source contains multiple data item types , the data member specifies which data item type to use 當數(shù)據(jù)源包含多個數(shù)據(jù)項類型時,數(shù)據(jù)成員指定使用哪個數(shù)據(jù)項類型。
A join operation on two relations which share a common data item type produces a combined relation with attributes specified in the join operation 對共享一個公共數(shù)據(jù)項類型的兩種關(guān)系,連接操作產(chǎn)生一種復(fù)合的關(guān)系,該關(guān)系具有連接操作中規(guī)定的屬性。