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ER Diagram - Cardinality
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?
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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


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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

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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

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

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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.
