This is personal study note Copyright and original reference: https://www.youtube.com/watch?v=fKKpdwSlQ2U&list=PLVsNizTWUw7HhYtI-4GGmlJ5yxNdwNI_X ================================================================================ ================================================================================ 1:N relationship When you send "query", you would like to send "query" with joining these 2 tables ================================================================================ Inner join = (ordinary) join = most used join To make statement clearer, To make statement simpler using alias, ================================================================================ ================================================================================ Result shows only the members who bought the product But you want to make the result which shows all members For that case, you can use outer join ================================================================================ Join 3 tables student table, club table: N:N relationship N:N is actually concatenation of 1:N ================================================================================ ================================================================================ ================================================================================ On --> for 1:N relationship ================================================================================ ================================================================================ ================================================================================