Question of MySQL and the KEYs

[Anon]

Can someone explain to me what the Primary KEY does and what the unique id does?

Also, how would I make a call to a MySQL database to find the highest number in a specific field.. i.e. records sorted by numbers, how would I query the database to find out what the highest record number thus far was?

Thanks in Advance

[Anon]

a primary key is a unique identifier.

Thus you can reference a whole row if you know what is it s specific id.

This id may be used in another table (being then a foreign key) to associated a row with a row from another table.

Lets imagine a table:
USERS
id (primary key)
user (name)
mail (email)

1 hendrix [e-mail]
2 bilbo [e-mail]
3 bruce lee [e-mail]

INTEREST
And another one:
id primary key
name

1 music
2 genealogy
3 pot
4 sport

You will be able to associate some interest
with some users by another table that will use 2 foreign key (one pointing to users, the other to interets):

1 id
2 user_id
3 interest_id

1 1 1
2 1 3
3 2 2
4 2 2
5 3 4

Thus you will be able (if needed) to make some sentence that will give you the email of every people interesyed in say sport, or whatever.

About the max trouble, mysql documentation may help you:

manual.html#example-Maximum-column
SELECT MAX(id) AS id FROM USERS

Another way to handle the troubles of knowing the latest row inserted (beware of someone inserting a row at the same moment you do) would be to use mysql_insert_id function.