ER Diagram - Cardinality

[Anonymous]

Hi All,

If there is 1:M (one to many join) between two tables (say Table A  to Table B) and Table A is one 1 side of 1:M then should this mean that Table A MUST ALWAYS HAVE AT LEAST 1 row?

[marcus_sommer]

I think, no - it musn't then it will be depend on the dataquality if all values have their versus-values within the other table. But best is you elaborate your question with a concrete issue.

- Marcus

[maxgro · Accepted solution]

In an ER diagram you have 

Maximum Cardinality

One-to-One [1:1]  

One-to-Many [1:N]  

Many-to-Many

the maximum number of entity instances that can participate in a relationship

Minimum cardinality

minimum number of entity instances that must participate in a relationship. 

zero [0] optional 

one [1] mandatory

Regarding your question I think the answer is no. 

You know you have a one to many relation (maximum cardinality);

you don't know if the relationship is optional or mandatory (minimum cardinality):

you can have 0 or 1 rows in TableA and for each row in TableA 0, 1, N rows in TableB

[Anonymous]

Thanks guys... your comments perfectly makes sense and clear the clouds

[Anonymous]

Hey .. one more clarification in the same context please -

if I have a mandatory one(denoted by 2 vertical small lines), should this mean that the table MUST HAVE at least 1 record/row . It may sound silly but I am just wondering if it is possible that the tables are empty (no records/rows) but still joined ?

I am more confused because such types of options are there in certification exam and I have already failed once by only 4% , got 66% and I found these types of questions very confusing.

Appreciate your response.