Why should use use char() for social security numbers?  Are characters ('123') somehow more space efficient than numbers (123)?

Now that we have seen varchar(), is there any place where it would be the best choice to use a string as the type rather than a varchar()? They seem functionally very similar to me, but varchar() I think are more space efficient?

If you are using varchar and you know that the information going inside a certain column will be around a general length, should you do varchar of a little more than that general length, or should you always do varchar of a large number? Does it make a difference in terms of space and time if you do varchar(1000) vs. varchar(200)?

What is the purpose of ""not null"" in ""uid int not null auto_increment""?  At first I thought it was to tell auto_increment not to just let rows stand with NULL as the primary key.  But you didn't say ""not 0"", and I also saw you use ""uid int auto_increment"", so I am confused as to what the ""not null"" is doing.

Can you go over updating tables with autoincrement and sets?

So, in other words, auto_increment means autogenerated? Can we auto_increment ids that include letters or other characters or is it only applicable to ints?

I'm a little confused as to how last_insert_id() solves the problem described in the reading where if you use max(uid), you might get the wrong value if someone else added a user before you could type the command. Wouldn't the same thing happen if you did last_insert_id() after another user was making changes as well?

How does deleting values work with auto-increment? Will the next value added fill in the missing space or be given the next highest key value like normal?

What do we do if we want multiple datatypes for one column?

more examples regarding the difference between set and enum

Could you show another example about the difference between set and enum?

Enum sorts using numerical values/they sort the order they are given in the `enum` list. I understand the second part of that statement (the `cat` precedes `dog` makes sense), but not the first part . Could you explain further? 

Do changes in one table flow through to other tables in a database if some columns have the same name? 

How does updating a table look like when it shares information with multiple tables? Are there ways to keep track of the changes?