多个SQL表和外键

Question

I wanted to know what is the better approach (performance wise) when working with Yii and mysql tables:

1. 1 table with ALL the columns
2. multiple tables (distribute the columns across them) with foreign keys relations

basically, what i'm asking is: Is there any performance degradation when a single table has many relations since Yii will fetch all the corresponding tables into the object when you do a query?

A more general question is: what is good practice and smarter approach when dealing with 1 vs multiple tables and foreign keys (again performance..)

Answer 1

If performance was the only criteria in your design strategy, you would choose 1. A very bad choice, that would give you a lot of problems in a later stage of your problems.

Databases should be designed with normalization in mind, so without any doubt, you must choose option 2!

Update

Example of normalized and not normalized: suppose we have a table with dvd rentals, and want to keep track of who rented the dvd:

flat table:

CREATE TABLE DVD (
  DvdId INT NOT NULL AUTO_INCREMENT, 
  DvdTitle varchar(64),
  Rental1 varchar(64), 
  Rental2 varchar(64), 
  Rental3 varchar(64),
  Primary Key(DvdId)
);

There is room for 3 rental entries, after that there is trouble. Beside that, when there is a extra field needed for the phone number of the customers, 3 extra fields must be made.

First step normalization:

CREATE TABLE DVD (
  DvdId INT NOT NULL AUTO_INCREMENT, 
  DvdTitle varchar(64),
  Primary Key(DvdId)
);

CREATE TABLE Rentals (
  RentalId INT NOT NULL AUTO_INCREMENT, 
  DvdId INT NOT NULL, 
  CustomerName varchar(64),
  RentalDate DateTime,
  Primary Key(RentalId)
);

This is better, but still not fully normalized. What if a customer rents the same dvd twice. Then there would be a double customer entry in the rentals table.

Final:

CREATE TABLE DVD (
  DvdId INT NOT NULL AUTO_INCREMENT, 
  DvdTitle varchar(64),
  Primary Key(DvdId)
);

CREATE TABLE Rentals (
  RentalId INT NOT NULL AUTO_INCREMENT, 
  DvdId INT NOT NULL, 
  CustomerId INT NOT NULL, 
  RentalDate DateTime,
  Primary Key(RentalId)
);

CREATE TABLE Customers (
  CustomerId INT NOT NULL AUTO_INCREMENT, 
  CustomerName varchar(64),
  Primary Key(CustomerId)
);