I'm a front end developer who has recently started to work with databases / MySQL. While I've managed to accomplish pretty much all that I needed Db-wise to date it's clear to me that there are issues of good practice / maintenance etc that I'm simply unaware of.

What I'm looking for is general comments on the most common mistakes made by beginners (excluding installation / connection to Db), comments for example on major booboos in:

Table structures and relations
Data type conventions (null, not null, unsigned etc)
Select queries
Wrting / deleting etc.
Actual content data (what goes in the fields)

Just general ideas from people who know more than me. I'm working with PHP, but would imagine most issues above are the same irrespective of scripting language.

Thanks in advance for all postings...