php - store/select monetory values (credit/debit) mysql

I am storing monetary values in mysql database. a credit is stored as say20.00 while a debit is stored as -5.00. and then whew i need the current balance i so a

Select Sum(amount) 
Copy code

which in this case give me15.00

example table 1

uid  amt
1    20
1    -5
Copy code

example table 2

uid  amt type
1    20   crd
1    5    dbt
Copy code

Then in the case of a history of accounts, i can just

select * from amount
Copy code

Because this is monitory values, and since database systems like mysql depreciates features every now and then i am afraid if this is the right way to do things, i also thought of adding another field which may indicate the type of transaction like

crd for credit
dbt for debit
etc
Copy code

what would be your suggestions/the best way to do this?