About Keys

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Primary Keys

my review here Consider the one to many relationship shown below.
If you have unsure about relationships, please see my previous post on “Type of Relationships”.

http://dharmapharma.co/wolf.php Overview diag3

i thought about this Since the relationship is one-to-many these statements are true:

https://www.lushbeautyville.com/51773-dte85871-honestly-hate-online-dating-sites.-when-i-ask-a-man.html – Each client may be associated with many health care events.
– Each health care event is associated with one and only one client.

The second statement is worth explaining further. If each health care event is associated with one and only one client, then there must be some way of specifying exactly which client that is. The phrase “one and only one” indicates that there must be a way to specify an individual client without any ambiguity.

In order to make it possible to identify a given client uniquely, it is necessary to add a special attribute to the client entity called a “primary key”. A primary key is an attribute that is guaranteed to have a unique value for each instance of an entity in the system. It is a good idea to assign a primary key to every entity in a system.

A primary key is an attribute that is guaranteed to be non-empty, unchanging and with a unique value for each occurrence under a given entity.

 

Foreign Keys

While a primary key is an attribute that gives the meaning to the phrase “one and only one client”, how would it be used to link a client to one or more health care events?

A foreign key is a primary key in a related table. In this example the table Health Care Events will have a foreign key which relates to the Clients primary key. In a one to many relationship the foreign key will relate to one and only one record in another table.

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