[thelist] SQL Multiple entries in same column?

rudy Rudy_Limeback at maritimelife.ca
Mon Nov 13 14:38:08 CST 2000

> there are two types of relationships between data: 
> one-to-many, and many-to-many.

hi oliver

please allow me to expand on this a bit...

in a physical database there is only one type of relationship -- 

it is implemented through primary keys and foreign keys

the table with the primary key is on the "one" side of the relationship, 
while the table with the foreign key is on the "many" side

the "one" table is also often called the "parent" table, and the "many" 
table is the "child" table

multiple child rows can have the same value in their foreign keys,but 
there can only be one parent row for any single value of the primary key 

many-to-one is the same as one-to-many

okay, that's the physical model

from a logical or conceptual viewpoint, you will often hear about 
many-to-many relationships as well

indeed, many-to-many relationships are common in real-world applications

the thing to remember, though, is that a many-to-many logical relationship 
is always implemented as two one-to-many physical relationships

holler if you would like me to give examples

finally, there is also the rarer one-to-one relationship, but this is also 
implemented as a one-to-many physical relationship -- think about it, one 
of the tables will have a foreign key pointing to the other table, with 
perhaps an additional constraint to ensure the "many" table has unique 
foreign keys 

hope that helps a bit


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