Beginning SQL

Chapter 4
122
The Street, City, and State fields all have a transitive dependency on the ZipCode field. To comply with
third normal form, all you need to do is move the Street, City, and State fields into their own table,
which you can call the ZipCode table:
Field Name Data Type
ZipCode varchar(12)
Street varchar(100)
City varchar(75)
State varchar(75)
Then alter the MemberDetails table and remove the Street, City, and State fields, leaving the ZipCode
field as a way of matching the address details in the ZipCode table:
Field Name Data Type
MemberId integer
FirstName nvarchar(50)
LastName nvarchar(50)
DateOfBirth date
ZipCode varchar(12)
Email varchar(200)
DateOfJoining date
The advantages of removing transitive dependencies are mainly twofold. First, the amount of data
duplication is reduced and therefore your database becomes smaller. Two or more people living on the
same street in the same city in the same town have the same zip code. Rather than store all that data
more than once, store it just once in the ZipCode table so that only the ZipCode is stored more than
once.
The second advantage is data integrity. When duplicated data changes, there’s a big risk of updating
only some of the data, especially if it’s spread out in a number of different places in the database. If
address and zip code data were stored in three or four different tables, then any changes in zip codes
would need to ripple out to every record in those three or four tables. However, if it’s all stored in one
table, then you need to change it in only one place.
There is a downside, though: added complexity and reduced efficiency. Before changing the database,
the query for a member’s name and address would be as follows:
SELECT FirstName, LastName, Street, City, State, ZipCode
FROM MemberDetails;