What is a common way to save 'debit' and 'credit' information?
Another way to do this (no idea if this is common or not!):

create table `transactions` (
  ... some columns ...
  amount decimal(10,2),
  ttype int,
  foreign key (ttype) references transactiontypes (id)
  ...
);

create table transactiontypes (
  id int primary key,
  description varchar(30),
  multiplier int
);


And then in transactiontypes you have:    

select * from transactiontypes;

+----+---------------+------------+
| id | description   | multiplier |
+----+---------------+------------+
|  1 | deposit       |          1 |
|  2 | withdrawal    |         -1 |
+----+---------------+------------+


And possibly more rows.

Then all amounts in transactions will be positive, and a join to transactiontypes + multiplication by multiplier gets the "real" (positive/negative) amount.