This is personal study note Copyright and original reference are from: https://www.youtube.com/watch?v=RHrp8xsgVr8&list=PLuHgQVnccGMDF6rHsY9qMuJMd295Yk4sa ================================================================================ You should consider "cardinality" and "optionality" elements in terms of relationship ================================================================================ Cardinality: "number order" like 1,2,3,... ================================================================================ "One homeroom teacher" can have "one classroom" "One classroom" can have "one homeroom teacher" Homeroom_teacher table id name class_id 1 kim 1 2 lee 2 3 park 3 Class table id name homeroom_teacher_id 1 1class 1 2 2class 2 3 3class 3 ================================================================================ Above 1:1 relationship can be represented by ================================================================================ "One author" can write "multiple replies" Author : Reply (N) ================================================================================ "One reply" is written by "one author" Author (1) : Reply (N) ================================================================================ Author id name 1 kim 2 park 3 lee Reply id contents author_id 1 hi 1 2 good 1 3 curious 2 4 bad 3 ================================================================================ Therefore, above relationship becomes 1:N relationship And, in ERD, 1:N relationship can be represented by Some ERD uses "1 and N notation" over the line without using "trident shape" ================================================================================ Suppose the scenario where "one text" can be written by "multiple authors" like "coauthor" ================================================================================ "One author" can write "multiple texts" Author : Texts (M) ================================================================================ "One text" can have "multiple authors" Author (N) : Texts (M) ================================================================================ In ERD, N:M relationship can be represented by