02 【高级SQL技巧】之【互为好友】&经典题目【互为好友】

1.题目

user_id|friends
1001|1002,1003,1004,1005
1002|1001,1007,1008,1010
1003|1002,1004,1006
1004|1005,1006,1008
1005|1002,1001,1004
1006|1002,1003,1004,1007
1007|1004,1010
1008|1001,1002,1003,1004
1009|1005
1010|1001,1003,1004,1005
–请计算有多少对互为好友的朋友

2.数据

with cte as (
          select '1001'  as  user_id,'1002,1003,1004,1005' as friends
union all select '1002'  as  user_id,'1001,1007,1008,1010' as friends
union all select '1003'  as  user_id,'1002,