This is personal study note Copyright and original reference are from: https://www.youtube.com/watch?v=N9NeNEhwGBY&list=PLuHgQVnccGMDF6rHsY9qMuJMd295Yk4sa ================================================================================ Option: existing is okay, none-existing is also okay ================================================================================ Registered "author" can have "no reply" Author id name reply_id 1 kim 1,2,3 2 lee 3 park Reply id content author_id 1 hi 1 2 good 1 3 bad 1 To Author, Reply is optional ================================================================================ Optional relationship is represented by (O from Optional) ================================================================================ Each "reply" must have "author" Author id name reply_id 1 kim 1,2,3 2 lee 3 park Reply id content author_id 1 hi 1 2 good 1 3 bad 1 To Reply, Author is mandatory ================================================================================ Author (1, mandatory) : Reply (N, optional) ================================================================================ Mandatory relationship is represented by (| from Mandatory) ================================================================================ There is cardinality (1:1,1:N,N:M) along with optionality (mandatory:optional) ================================================================================