Question: Why this favour of function definition don't work?

I define a func

f(x,y):=piecewise(x=0 and y=0,139 ,x=1 and y=0,144,x=2 and y=0,149,x=3 and y=0,155,x=4 and y=0,155,x=5 and y=0,155,x=6 and y=0,155,x=7 and y=0,155,x=0 and y=1,144,x=1 and y=1,151,x=2 and y=1,153,x=3 and y=1,156,x=4 and y=1,159,x=5 and y=1,159,x=6 and y=1,159,x=7 and y=1,159,x=0 and y=2,150,x=1 and y=2,155,x=2 and y=2,160,x=3 and y=2,163,x=4 and y=2,158,x=5 and y=2,156,x=6 and y=2,156,x=7 and y=2,156,x=0 and y=3,159,x=1 and y=3,161,x=2 and y=3,162,x=3 and y=3,160,x=4 and y=3,160,x=5 and y=3,159,x=6 and y=3,159,x=7 and y=3,159,x=0 and y=4,159,x=1 and y=4,160,x=2 and y=4,161,x=3 and y=4,162,x=4 and y=4,162,x=5 and y=4,155,x=6 and y=4,155,x=7 and y=4,155,x=0 and y=5,161,x=1 and y=5,161,x=2 and y=5,161,x=3 and y=5,161,x=4 and y=5,160,x=5 and y=5,157,x=6 and y=5,157,x=7 and y=5,157,x=0 and y=6,162,x=1 and y=6,162,x=2 and y=6,161,x=3 and y=6,163,x=4 and y=6,162,x=5 and y=6,157,x=6 and y=6,157,x=7 and y=6,157,x=0 and y=7,162,x=1 and y=7,162,x=2 and y=7,161,x=3 and y=7,161,x=4 and y=7,163,x=5 and y=7,158,x=6 and y=7,158,x=7 and y=7,158)

It is really a chore . By the way , is there more concise way to definition of the same effet as this ? 

then I type:

sum(f(0, y), y = 0 .. 7) 

Maple just respond me a 0 

why ?