If you need to make negative numbers positive in SQL then the best way to achieve this is to use the built in maths function ABS which will return the absolute (positive) number of a numeric value.
To use the ABS function you simply call it and pass to it a numeric and the return value will be the positive representation of that number, so in the below we use the ABS function to convert a negative value
SELECT ABS(-102.34) -- RESULT = 102.34
If you were to use ABS on an already positive number then the result will be the same as the input ie the value won't change, in the below example passing the value 53 to the function will also return 53
SELECT ABS(53) -- RESULT = 53
For more information on the ABS function then you can check out the related MSDN page here MSDN ABS.