One of the worst database design that you have ever encountered was couple of years back when you worked as an intern in a small software firm.
I was testing a database that was designed for a local school where the data of students and parents were kept. The whole application was built on Java. The frontend was built using jQuery, database was connected with hibernate and backend was on spring sts. Database was mySql.
Like any normal relational database, it consists of rows and columns. The primary key that was used to for this database was telephone number of the parents and foreign key was last name of their kids. Now I was doing a normal test of the database I was found that were some major issues with the design of the database because the selection the primary key and foreign key were very bad.
1. Issues with primary key: Primary key was the phone number of the parents but unfortunately there were two identical numbers for two different people (it's an old number a parent which he/she disabled and took a number and old number was reassigned to another parent in the same school) so every time I executed a query with that primary key there was issue and the notifications were going to both parents instead of one. So had to change the primary key of the database to email addresses of the parents.
2. Issues with the foreign key: Just like I have mentioned about the issues in primary key there were identical last names of the many students. So I had to change the primary key of students database to their student ID and map it to parents database.
So, this was one very bad database design that I have worked with till now.