Add the specified members to the set stored at key.
Specified members that are already a member of this set are ignored.
If key does not exist, a new set is created before adding the specified
members.
An error is returned when the value stored at key is not a set.
@return
@integer-reply: the number of elements that were added to the set, not including all the elements already present into the set.
@history
>= 2.4: Accepts multiplememberarguments. Redis versions before 2.4 are only able to add a single member per call.
@examples
SADD myset "Hello"
SADD myset "World"
SADD myset "World"
SMEMBERS myset