Database design skills 2_PHP tutorial

1. Create an independent table for fields applied to multiple records
2. Associate the values ​​of these tables through a foreign key

We put the value of the url in an independent table, so that we You can add more data later without worrying about duplicate values. We also associate these fields with
primary key values:
users
userId name company company_address
1 Joe ABC 1 Work Lane
2 Jill XYZ 1 Job Street
urls
urlId relUserId url
1 1 abc.com
2 1 xyz.com
3 2 abc.com
4 2 xyz.com
As shown above, we have created a separate table, The primary key userid in the users table is now associated with the foreign key relUserId in the url table. The current situation
seems to have been significantly improved. However, what if we want to add an employee record to Company ABC? Or more, 200? This way we have to
reuse the company name and address, which is obviously not redundant enough. Therefore, we will apply the third-level normalization method:
The third-level normalization form
1. Eliminate the fields that do not depend on the key
The company name and address have no relationship with the User Id, so Their applications have their own company IDs:
users
userId name relCompId
1 Joe 1
2 Jill 2
companies
compId company company_address
1 ABC 1 Work Lane
　2 > In this way, we associate the primary key comId in the companies table with the foreign key named relCompId in the users table. Even if ABC company adds

200 employees, there will only be one record in companies. Our users and urls tables can grow continuously without worrying about inserting unnecessary data. Most
developers believe that three steps of formalization are enough. The design of this database can easily handle the burden of the entire enterprise. This view is correct in most
cases. .
We can pay attention to the fields of the URL-have you noticed the redundancy of data? If the HTML page for users to enter these URL data is a text
box, and they can input anything, there is no problem. The probability of two users entering the same favorites is less. However, if it is through a drop-down Menu, only let
user select two url inputs, or a little more. In this case, our database can also undergo the next level of optimization - the fourth step. For most developers, this step is ignored because it relies on a very special Relationship - a many-to-many relationship, which we have never encountered in our application.

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TechArticle1. Create independent tables for fields applied to multiple records 2. Associate these tables through a foreign key We put the value of the url in a separate table so that we can...