Well there's a couple of things to get you started. First off, the language is SQL and MySQL is a product which uses this language. That should help when searching for resources on it. You should check out RDBMS principles in general before you attempt to learn the language. Some of the functions won't make any sense until you understand how tables relate, primary/foreign keys etc.
With regards to your question about selecting and then updating...it's best not to think of things in such a linear way. SQL is quite advanced and isn't your usual shopping-list basic scripting. Why select and the update? Why not use the
where condition to specify what you want updated?
Code:
update address_table set post_code = 'AA11AA' where line_4 like '%dublin%'
Your reference to index numbers....well you're halfway on the right track. Don't depend on a random index. Look at the data you have and try to find a key. Say for instance if it was bills and invoices, you could use the invoice number. If it was customers, it could be a customer number. Hell it could even be a firstname or surname if you found it to be suitable and an individual idea.
As I said above it's really a subject you should get a good feeling for and understanding of relational databases before jumping in. The hello_world of SQL doesn't even scratch the surface
