How do I calculate the difference

I have data in two columns. I need to write a query that only shows the customers that 开发者_运维问答have gone over their credit limit and by how much:

customer balance         credit limit  
418.75                   500
10.75                    200
200.1                    100

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Of course this doesn't take customer information into account since you didn't mention any customer tables, but this should get you the customer balance and overage:

SELECT customer_balance, credit_limit, customer_balance - credit_limit as overage
FROM your_table
WHERE customer_balance > credit_limit

It's pretty straight-forward; you select customers with a balance over their limit, and just select the difference as a third column.