Question

I am designing a database structure for a mail list
The data i need to record is the mail receiver data:

Since the mail receiver data is not predefine,
it cause a problem:
for example:
some of the receiver have name, mail address, phone no
while the others have name, birthdate, mail address, gender, etc…

So my design is like this:
Table Receiver:
PK: ReceiverID
    AttributeID
    DataID

Table Attribute:
PK: AttributeID
    AttributeName

Table Data:
PK: DataID
    DataContent

A example of one receiver is:
Receiver: 1 1 1
Attribute: 1 mailAddress
Data:1 [e-mail]

Receiver: 1 2 2
Attribute: 2 Name
Data:2 Tony

This cause a problem because i want to check the data field type and length, what is a better database design? Sorry about the poor description, i would clarify it if you are confused .Thanks a lot.

Answer 1

Its impossible per definition to define datatypes for unknown attributes. This is just how databases work.

I understand you want the birthday to be a DATETIME column. (and possibly other “special” datatypes for a few other attributes)

I cannot believe the impossiblity to predefine the available attributes – they are limited. just go ahead and define a list of possible attributes.

A better database design would be to just create a receiver table that contains all possible fields and allows them to be NULL. Using the database as a “generic storage” for arbitrary attributes is bad practice when not necessery.

your table could look like this:

CREATE TABLE maillist_receivers (
  id INT NOT NULL AUTO_INCREMENT, 
  PRIMARY KEY(id),
  email VARCHAR(128) NOT NULL, 
  firstname VARCHAR(128), 
  lastname VARCHAR(128), 
  phone VARCHAR(128), 
  birthdate DATETIME, 
  ...
)

And your data could look like this:

id     firstname      lastname     email            phone      birthdate
1      Max            Power        [e-mail]       NULL       21.12.2012
2      John           Doe          [e-mail]     555-1234   NULL

• It performs better
• the datatypes are well defined
• You can check in your application code if a value is NULL and handle that accordingly. (as in: do not send birthday card, ..)
• it is clean – you can read the plain data in a database browser