Help with a trigger to change field to a negative value

[mark paterson]

I have a database called check_register that has one table called transactions. The transactions table has the following columns:

transID int (auto increment) primary key
amount int
withdraw varchar [values are either yes or no]


is there a way to write a trigger that will look to see if the withdraw field = yes then change the amount value to a negative value?



for example, a record entered with the following values:

transID: 1
amount: 20.00
withdraw: yes

the trigger would replace 20.00 with -20.00



Thank you in advance. I'm trying to teach myself ways to have the database help with data manipulation,