 Usage:

SDIFF key [ key ... ]
 Complexity:
 O(N) where N is the total number of elements in all given sets.
 Since:
 1.0.0
Returns the members of the set resulting from the difference between the first set and all the successive sets.
For example:
key1 = {a,b,c,d}
key2 = {c}
key3 = {a,c,e}
SDIFF key1 key2 key3 = {b,d}
Keys that do not exist are considered to be empty sets.
Examples
127.0.0.1:6379> SADD key1 "a"
(integer) 1
127.0.0.1:6379> SADD key1 "b"
(integer) 1
127.0.0.1:6379> SADD key1 "c"
(integer) 1
127.0.0.1:6379> SADD key2 "c"
(integer) 1
127.0.0.1:6379> SADD key2 "d"
(integer) 1
127.0.0.1:6379> SADD key2 "e"
(integer) 1
127.0.0.1:6379> SDIFF key1 key2
1) "a"
2) "b"