How important are constraints like NOT NULL and FOREIGN KEY if I'll always control my database input with PHP?

Question

I am trying to create a column in a table that\'s a foreign key, but in MySQL that\'s more difficult than it should be. It would require me to go back and make certain changes

Answer 1

Implement default values and constraints at the database level; rules that will result in acceptable data to any consuming application. This insulates you from integrity issues.

Then, implement better default values and constraints at the application level. If you are prohibited technically (access to APIs external to the database) from implementing a constraint, or generating a default value at the database level, the application is where you'll need to do it. This will result in a better default value. However a segfault, or general failure of the application will not result in unacceptable data being persisted.