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# relation between two tables/views

Posted on 2012-04-12
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hi guys

In my ER diagram i see

table1    table2

what does the arrow mean ?

thanks
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Question by:royjayd
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Accepted Solution

sdstuber earned 137 total points
ID: 37837578
assuming the arrow is from table1 to table2

table1 has a many to one relationship with table2

so table2 is likely the parent of table1
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Assisted Solution

gnivkor earned 25 total points
ID: 37837626
that there is a relationship to that table through some sort of constraint, maybe primary / foreign or unique
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Assisted Solution

Milleniumaire earned 63 total points
ID: 37838045
The fact that the line is dotted means the relationship is optional.  The small vertical line through the dotted line implies that the key of table1 (table on the left) is part of the key of table2 (on the right).

I've never seen an ER diagram that uses arrows on the end of the lines and I can only guess that this is suggesting the direction of the relationship, as described by sdstuber.
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Expert Comment

ID: 37838123
Partial correction to gnivkor above, the relationship is one or more columns in column, commonly called a parent to child relationship.  IOW for any given parent table row, there MAY be one or more child table rows.  Constraints are required to enforce relational integrity -- but they are not required.

royjayd, here's a reference to explain the different relationships shown by crow's-foot notation:  w2.cs.uregina.ca/~bernatja/crowsfoot.html

HTH,
dvz
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Author Comment

ID: 37838284
My inititial understanding in laymans terms: The table which has primary key (one)  --> maps to table in which that primary key is used as a foreign key (many).
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Assisted Solution

David earned 125 total points
ID: 37838316
Nods, the parent shares a family name with her/his child(ren).  In this example, there would be a primary (unique) key constraint on the parent, and that value is also stored in each child.  For RI you define a foreign key in the child table, referencing the parent key value.
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Expert Comment

ID: 37838323
Yes, that is correct in terms of primary and foreign key references.  In terms of the ERD, the many side of the relationship is shown with a "crows foot" and the one side with a single line.
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Author Comment

ID: 37856989
is Many to One relationship just flipping One to Many relationship ? or is there anything extra to it? For example the above arrow can be used to represent Many-to-One and
One-to-Many relationship aswell, right?

thanks
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Assisted Solution

sdstuber earned 137 total points
ID: 37857004
yes

I used the phrase "many to one" based on the direction of the arrow.

but had it been pointing the other way I'd have said "one to many"
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Assisted Solution

David earned 125 total points
ID: 37857294
Different notations are used for other business rules beyond 1-M, roy.  In your example the bisecting line shows one-or-more.  Replace the "1" line with "0" and (no surprise) it represents zero or more.  The order in which the entities/tables are described may be flipped (sbstuber above, 37867004).

See if this example helps:  a business must have one and only one tax ID number, and a tax ID number is unique to one business.  However, a business must have at least one address; that business MAY have multiple addresses; and an address MAY be used by one or more businesses.
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