How to undo an sql statement?

Answer 1

You need to learn the concept of transactions.

First of, before doing work, you can set a savepoint, remember it wont be saved, its just a point.

savepoint s1;

Later do some work, and if you like you can save it using :-

commit; or commit s1;

If you want to undo,

rollback to savepoint s1;

NOTE :- You need to start a transaction first,use
begin;

Also, if you do a thing, any random code, it automatically gets saved.Thats because autocommit is "on" by default. Use this :-

set autocommit = false;

(I DONT KNOW THE EXACT LINES, YOU CAN GOOGLE THEM)

Answer 2

do an other SQL statement.
in other words, you cant undo drop tables.