table maintanace

hi

i have create one Z table in that i have 3 fileds .

whenever user insert the data 2 fileds data should be unique.

the table i have created as below .

Z_CABJN Z_VBELN Z_POSNR

x 1000 10

y 1000 20

z 1000 30

my table shou not accept

x 1000 40

for ever change of Z_VBELN / Z_POSNR i sholud have new

Z_CABJN.