sets类型及操作：
set是集合，它是string类型的无序集合。set是通过hash table实现的，
添加、删除和查找的复杂度都是O(1).对集合我们可以取并集、交集、差集。
通过这些操作我们可以实现sns中好友推荐（如QQ）和blog的tag（标签）功能。
sadd方法：添加元素
smembers方法：查看集合的所有成员
127.0.01:6379> SADD set "one" "two"
(integer) 2
127.0.01:6379> SMEMBERS set
1) "two"
2) "one"
srem方法：删除集合元素
127.0.01:6379> SMEMBERS set
1) "two"
2) "one"
127.0.01:6379> SREM set "one"
(integer) 1
127.0.01:6379> SMEMBERS set
1) "two"
spop方法：随机返回并删除名称为key的集合元素
127.0.01:6379> sadd set "two"
(integer) 0
127.0.01:6379> sadd set "one"
(integer) 1
127.0.01:6379> sadd set "three"
(integer) 1
127.0.01:6379> sadd set "four"
(integer) 1
127.0.01:6379> sadd set "five"
(integer) 1
127.0.01:6379> spop set
"five"
127.0.01:6379> spop set
"one"
127.0.01:6379> spop set
"three"
127.0.01:6379>
sdiff方法：两个集合的差集
127.0.01:6379> sadd set1 "one" "two"
(integer) 2
127.0.01:6379> sadd set2 "three" "two"
(integer) 2
127.0.01:6379> SMEMBERS set1
1) "two"
2) "one"
127.0.01:6379> SMEMBERS set2
1) "two"
2) "three"
127.0.01:6379> SDIFF set1 set2
1) "one"
127.0.01:6379> SDIFF set2 set1
1) "three"
sdiffstore方法：将两个集合的差集存到一个集合中
127.0.01:6379> SMEMBERS set2
1) "one"
2) "two"
3) "three"
127.0.01:6379> SMEMBERS set1
1) "three"
127.0.01:6379> SDIFF set1 set2
(empty list or set)
127.0.01:6379> SDIFF set2 set3
1) "three"
2) "two"
3) "one"
127.0.01:6379> SDIFF set2 set1
1) "two"
2) "one"
127.0.01:6379> SDIFFstore set3 set2 set1
(integer) 2
127.0.01:6379> SMEMBERS set3
1) "two"
2) "one"
sinter取两个集合的交集：
127.0.01:6379> sinter set1 set2
1) "three"
sinterstore 返回所有给定key的交集，并将结果存到另一个key
127.0.01:6379> sinterstore set4 set1 set2
(integer) 1
127.0.01:6379> SMEMBERS set4
1) "three"
sunion返回所有给定key的集合的并集
127.0.01:6379> sunion set1 set2
1) "two"
2) "one"
3) "three"
sunionstore 将并集存储到另一个集合中
127.0.01:6379> sunionstore set5 set1 set2
(integer) 3
127.0.01:6379> SMEMBERS set5
1) "two"
2) "one"
3) "three"
smove方法：从第一个key对应的set中指定元素移动到第二个集合中
127.0.01:6379> SMEMBERS set5
1) "two"
2) "one"
3) "three"
127.0.01:6379> smove set5 set6 one
(integer) 1
127.0.01:6379> SMEMBERS set5
1) "two"
2) "three"
127.0.01:6379> SMEMBERS set6
1) "one"
scard方法：查看集合中的元素个数
127.0.01:6379> SMEMBERS set5
1) "two"
2) "three"
127.0.01:6379> scard set5
(integer) 2
sismember方法：测试指定的元素是否是集合中的元素
127.0.01:6379> SMEMBERS set5
1) "two"
2) "three"
127.0.01:6379> SISMEMBER set5 "two"
(integer) 1
srandmember方法：随机返回名称为key的set的一个元素，但不删除元素
127.0.01:6379> SMEMBERS set5
1) "two"
2) "three"
127.0.01:6379> SRANDMEMBER set5
"three"
127.0.01:6379> SRANDMEMBER set5
"three"
127.0.01:6379> SRANDMEMBER set5
"three"
127.0.01:6379> SRANDMEMBER set5
"two"