Can a column have more than one datatypes in it?

Why does MySQL prevent duplicate entries when using a primary key, but not when using a non-primary key column?

What happens to existing rows in a table in the instance where you need to update a column, e.g, change name?

Are enum types case sensitive? By this, I mean when inserting rows into a table, if the case does not match the enum options for a column, will you get an error?

If you are making a database table and know a column will use an integer value, but you aren't sure how big they will get, what should you use as the default datatype? What happens if you try to store a value larger than 255 in a tinyint datatype column?

Can you change a column's datatype once the table is already created? If so, what happens to the values already in the column?

Is there a way to automate an "are you sure" check to avoid accidental database deletion when something like "delete from {db};" is run?

Can you explain the line 'uid int not null auto_increment'? Why does it auto increment on 'not null' instead of 'null'.

Why would we not always just use varchar for strings like emails, passwords, etc..

Why is fixed-length data faster to access (in the context of char versus varchar)?

If SQL has no "undelete", is there any other way that we could possibly recover some mistake made??

How to dump existing data to sql database?

None so far! / no questions, thank you! / None! This builds nicely on our current SQL knowledge.