If you run your APEX application - like a Form based on the EMP table - and test if you can change the value of Department to something else then the standard values of 10, 20, 30 or 40, you'll get a nice error message like this: But it isn't really nice, is it? So what do a lot of developers do? They create a validation (just) in order to show a nicer, better worded, error message like "This is not a valid department". And what you then just did is writing code twice : Once in the database as a (foreign key) check constraint and once as a sql statement in your validation. And we all know : writing code twice is usually not a good idea - and executing the same query twice is not enhancing your performance! So how can we transform that ugly error message into something nice? By combining two APEX features: the Error Handling Function and the Text Messages! Start with copying the example of an Error Handling Function from the APEX documentation. Create this function
Almost every APEX Developer knows that JavaScript and CSS belongs in separate files and (in 99% of the use cases) not somewhere in your Page properties. If you have this code in separate files it is easier to use them in a Version Control System (SVN or Git) - if the files are outside of APEX. And the code you store in these files can be reused, in contrast to the stuff you store on Page level. In a lot of environments it is harder to use external files, because deployment of these files to the appropriate location on a webserver requires special privileges that not everybody has. In those cases storing these files as Static Application Files or Static Workspace Files might be a better solution. It makes deployment easier, because these files will be exported and imported as part of the application. You can't accidentally forget about them. But working with these files is quite a pain. When you need to edit something you have to download that file, make the changes, upload it again