Saving Data on Android

foreign key
 that 
references a primary key of another table. For example, say that you have an 
Orders
 
table that stores the number of Blu-ray sales for each movie like below:
In this case, the 
MovieID
 is a foreign key that identifies a unique movie record in 
your Movies table. Therefore, if you want to retrieve extra details about an available 
movie, you would only need to retrieve the movie row from your Movies table with 
that particular ID. As such, you don't have to store all of the data about ordered 
movies in the 
Orders
 table, you can just look up movie details using the foreign key. 
This makes each order object very light while holding enough data to look up more 
detailed information.
Although not all the database management systems will force you to have a primary 
key, it is strongly recommended that you have one for each of your tables since it 
makes it much easier to retrieve information using queries. But, usually, when 
working with databases, you can specify you want a primary key, and it gets auto-
generated, so you don't have to worry about it.
Rows
 represent a record in your database and can contain as many columns as 
needed to represent your data.