mysql summation with condition on individual rows -

i have table 3 fields (of relevance question). 1 field numeric, other 2 have text-based content. want sum of numeric field follows:

1. group field a
2. if field b equal x, add

3. if field b equal y, subtract

   abc, x, 25
   abc, x, 15
   abc, y, 10
   def, x, 20
   def, y, 5

the above data return:

abc, 30 def, 15 

my query, ideally, produce equivalent of following following:

select fielda, sum(fieldc) sum1 my_table fieldb = 'x' group fielda select fielda, sum(fieldc) sum2 my_table fieldb = 'y' group fielda  result = sum1 - sum2 

how can in single query?

something should work:

select   fielda,   sum(case when fieldb = 'x' fieldc else -fieldc end) result my_table group fielda