table x
column  "month" "customer"
month  customer
1         1
1         2
1         3
1         4
2         1
2         2
2         3
2         4
2         5
3         1
3         2
3         3
3         4
3         5
3         6


now i want to delete the customers in second and third month who are all not in first month

please tell me