Question : SQL query

I am writting a SQL query. Below is the sample table. The result i want is for each customer, difference of sum of by and sum of sell.

Table:

customer     trtype      amount
0001            by           10000
0001            by            2000
0001            by            500
0001            sl             3000
0001            sl             2000  
0002            by           60000
0002            by            4000
0002            by            500
0002            sl             3000
0002            sl             2000


example:
0001 :   (10000+2000+500) - (3000+2000) = 12500-5000 = 7500
0002 :   (60000+4000+500)-(3000+2000) =  64500-5000 = 59500


Result should be

0001 - 7500
0002 -  59500

Answer : SQL query

try this

Select A.customer, B.Name,  sum(case when trtype='by' then amount else -amount end)
from tablename A join customerTable B
on A.customer = b.code
group by A.customer, B.Name