Can you finish all these SQL injection challenges?

My friend created a website where we can store secrets... Unfortunately, we can only see our own. Help me find all of my friend's secrets.

I think an administrator blocked my account. Can you help me steal someone else's account?

Note: There are two flags in this challenge.

Time for bug bounties! This multinational technology company has been hacked so many times, we might be able find a new bug and make some money out of it...

Seems like our previous bug has been patched now... Let's double check that they did their job properly.

These developers are sloppy... They claim that their website is secure now (for the second time), I hope they are right this time...

I'm starting to believe that these developers are complete idiots. They just claimed that blacklisting single-quotes and double-quotes solve every SQL injection issue.

Let's prove them wrong.

My teacher has this weird website. I doubt there's any useful information in the database. Maybe we can leak the /etc/passwd file instead?