Database Diagram - How to interpret the symbols that explain table relationships

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Database Diagram - How to interpret the symbols that explain table relationships

meichner

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January 5, 2010 at 9:09 am

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I hate to ask such a simple question, but I have not been able to find out the meaning behind the lines and symbols that represent the relationships between two tables. When looking at examples of database diagrams I am trying to determine if the relationship between two tables is one to one, one to many or many to many. I would appreciate it if someone could either explain what these symbols mean or could point me to a resource that explains how to interpret these symbols.
For example, in a book that I am reading there is a diagram that shows two tables; TableA and TableB. There is a line between the two tables. At one end of the line, next to a field on TableA is the symbol <><>. At the other end of the line, next to TableB is a key symbol. Does this mean that there is a one to many relationship between TableA and TableB and that there are many TableA's to one TableB?
Thanks

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AndrewSQLDBA SSC-Insane Points: 21827 More actions · Answer 1

One the end of the line with the key, that is connected to the Primary Key, on the other end, in this one to many relationship, that is the oo (omega) or the Infinity symbol. Meaning that there are one or many numbers of records that are related.

This has been the norm for many decades of relational databases. This is nothing new to SQL Server 2008

Andrew SQLDBA

Ray K SSC-Dedicated Points: 31165 More actions · Answer 2

Andrew is correct. A key indicates a single relationship; an infinity symbol indicates a "many" relationship.

For example, a connection with a key at each end indicates a one-to-one relationship.

If it has a key at one end and an infinity symbol on the other, that is a one-to-many relationship.

Does that help?

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meichner SSCertifiable Points: 5563 More actions · Answer 3

Ray K (1/5/2010)
Andrew is correct. A key indicates a single relationship; an infinity symbol indicates a "many" relationship.
For example, a connection with a key at each end indicates a one-to-one relationship.
If it has a key at one end and an infinity symbol on the other, that is a one-to-many relationship.
Does that help?

Thanks so much to the both you.

Ray K SSC-Dedicated Points: 31165 More actions · Answer 4

No prob! 🙂

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Grant Fritchey SSC Guru Points: 398909 More actions · Answer 5

There are multiple web sites out there that list the various symbols used by the different modeling methods. For example, here's the class, probably most reductive example: http://en.wikipedia.org/wiki/Entity-relationship_model

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meichner SSCertifiable Points: 5563 More actions · Answer 6

Grant Fritchey (1/5/2010)
There are multiple web sites out there that list the various symbols used by the different modeling methods. For example, here's the class, probably most reductive example: http://en.wikipedia.org/wiki/Entity-relationship_model

Thanks Grant. I had actually found that example when I was trying to figure out the meaning of symbols that were used in the book I am reading. I was confused because the wiki page didn't contain the symbols oo and key (unless I somehow had missed it).

The meaning of the key was obvious. Now that I know that oo meant infinity, it too is pretty obvious.

I appreciate the help.