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Create table relationships (Access basics, part 2)
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Create table relationships (Access basics, part 2) Closed
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WizardUse the paneMany-to-manyOne-to-one3:191:313:253:065:07Intro
& types1/5 videos
Now that we have a set of tables, we can build a relationship
between them.Relationships are critical because they link your
tables back together after you divide your data,and also because
they have a big impact on how you design your queries and get
answers from your data.Any relational database uses three types of
relationships between its tables: one-to-many, many-to-many, and
one-to-one.The relationship you use depends on your data, and heres
what I mean by that.You have a one-to-many relationship when a
single record in one tableis related to one or more records in
another table.And our sample database has this kind of
relationship.One customer can have one or more phone numbers, and
each number is only related to one customer.You have a many-to-many
relationshipwhen a single record in table A can be related to one
or more records in table B,and a single record in B can also be
related to one or more records in table A.You typically see
many-to-many relationships in order tracking databases,where an
order can contain many products, and a product can be part of
multiple orders.Youll also see this type of relationship in student
databases, where many students can occupy many classrooms.You have
a one-to-one relationship when a single record in one tableis
related to just a single record in another table.For example, a
single employee may drive one company car.But you wont see this
very often, because one-to-one relationships arent very common.So
now I want to select Database Tools, then Relationships.That starts
the Relationships pane, and youll use this tool quite a bit.This is
where you build and manage most of your relationships,and this is a
typical relationshiptwo tables with a line between them.If you dont
see a line, the tables arent related.Whats more, this is a
one-to-many relationship,and you can tell it is because the
relationship line here has a 1 and an infinity symbol.And those
symbols point to a couple concepts you need to understand.First,
the table on the one side is a parent table, and the table on the
many side is a child table.Second, you can think of most of your
relationships as having sides.In this case, you have a table on the
one side and a table on the many side.Sides are important because
they have a big impact on how you design your queries,and well see
how that works in the next course.For now, just keep the idea of
sides in mind.So back to our relationship, and Im going to select
the relationship line,then Edit Relationships here on the
ribbon.This dialog controls three settings that you need to
understand in order to use relationships properly.Referential
Integrity, Cascading Updates, and Cascading Deletes can have a big
impact on your data, and heres what they do.First, Referential
Integrity synchronizes the records in both tables.This is a
critical part of keeping your data accurate, so you almost always
want to set it.For example, with referential integrity enforced, I
cant add a phone number unless I have a name in the Customers
table.That prevents me from creating what are called orphans, data
with no parent records, because orphans can cause problems.For
example, just imagine a sales figure with no order attached,and you
can see why Im harping about setting referential
integrity.Cascading Updates allows any changes on the parent sideto
cascade through to any related records on the child or many side.So
if a customer changes his name, for example, you only have to enter
that change in the Customers table,and any related records will
change.Cascading deletes control whether you can delete data. If
you dont select the option, you cant delete.But if you do, then
when you delete a record, Access deletes everything related to that
record.For example, delete a customer and the phone numbers go,
too.That can be a good thing because it prevents orphans, numbers
with no customer.But remember your business rules may prevent you
from deleting data, so keep them in mind.They can be an key part of
your database design.And finally, what weve covered here may seem
like a lot to absorb,but for the rest of this course, well put it
all to use and create each type of relationship, so keep going.
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Create table relationships (Access basics, part 2) Closed
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Lookup WizardUse the
paneMany-to-manyOne-to-one3:191:313:253:065:07Intro & types
One of the easiest ways to create a one-to-many relationship is
to use a tool called the Lookup Wizard.The wizard actually does two
jobs for you. It creates the relationship, and when youre done,the
foreign key field lets you choose values from the table on the one
side of the relationship.That makes it easier to enter data.In our
example, all I have to do is select a namethats the value on the
one sideand enter a phone number.But remember one thing: When you
use the wizard,you dont need a foreign key field in your child
table because the wizard creates it for you.If you already have a
foreign key, thats okay, go ahead and use it, but if not, just let
the wizard create one for you.So heres our sample database, without
the foreign key that we added in the previous course.To start,
click the header on a blank field, or click the header in your
foreign key,if you have one, choose Lookup & Relationship, and
the wizard starts.Use the first option, looking up values from
another table, and choose Next.The parent table is Customers, so
make sure thats selected and choose Next.We need to look up the
values in the Customer Name field, so move that over.This is the
calculated field we created in the first course.You saw me using
this field at the start of this video, and notice that it shows you
the entire name.That takes the guesswork out of choosing the right
person, or more generically, the right data.Data entry is faster
and more accurate that way, and thats why we create that type of
field.So choose next, and a sort order is optional, so I wont
bother with one, just choose Next again.As a rule, you want to hide
the key column because it just clutters up your database if its
visible, so choose Next.And here Ill name the field Customer
IDFK.That tells me the parent field is Customer ID, and FK reminds
me that this is a foreign key.So now Ill select Enable Data
Integrity and Cascade Delete.This is how you enable referential
integrity and cascading deletes.Select Finish, and theres your
field.Finally, I want to make a change to the relationship, and in
order to do that, I need to close the Phone Numbers table.You cant
change a relationship if one of the tables involved in that
relationship is open.So after the table is closed, I go to the
Database tools tab and select Relationships.Then I select the
relationship line, then Edit Relationships here on the ribbon,and
you can see that referential integrity and cascading deletes are
both selected.Ill also enable the middle option, Cascade Update
Related Fields, select OK, and were done.And up next, were going to
do this all again, but were going to do it with the Relationships
pane, so keep going.
5761234Course summary8Help
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Another way to create a relationship is with the Relationships
pane, and heres what it looks like.These are the tables involved in
a relationship, and the line tells you whether its a one-to-many
relationship or not.The thing to remember about this tool is it
doesnt create a lookup list for you,unlike the Lookup Wizard, but
thats not all bad.If you use queries to extract and process your
data, you need the relationship,but not the lookup list, because
the query provides the data you want to see.So to use the pane,
select Database Tools, then Relationship.If the pane is blank, you
can drag your tables from the Navigation Pane, like this.Now,
unlike the Lookup Wizard, your child table has to have a foreign
key with the same data type as the primary key,or a compatible data
type.So theres my foreign key, and to create the relationship, I
just drag the primary key over to the foreign key and let go.That
starts the Edit Relationships dialog,and I enable referential
integrity, cascading updates and deletes, and select Create.There
you go, a one-to-many relationship.And up next, well create a
many-to-many relationship because you find them in any order
tracking database.
Lookup WizardUse the
paneMany-to-manyOne-to-one3:191:313:253:065:07Intro & types
5761234Course summary8Help
Create table relationships (Access basics, part 2) Closed
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Lookup WizardUse the
paneMany-to-manyOne-to-one3:191:313:253:065:07Intro & types
Now that youve seen how to create a one-to-many relationship,
lets create a many-to-many relationship.And to do that, well grow
our database into a rudimentary order tracking system, with
Products and Orders tables.And as a review, we have a many-to-many
relationship between these tablesbecause a product may appear in
many orders, and an order can contain many products.So the first
step in building a many-to-many relationship is to create a new
table.This is called an intermediate table, and it will store the
data from the other tables involved in the relationship.You may
also hear this new table referred to as a linking or junction
table.The next thing you do is create a pair of one-to-many
relationshipsbetween the junction table and the other tables
involved.As part of that, you also need to make both of these
fields into the junction tables primary key.This is called a
composite key, and the thing to remember is you have to build them
in Design view.So, Create, Table Design, and theres a new, blank
table.Ill also open the Products and Orders tables this way, too,
and the first thing Ill do is look at the primary keys for each.The
Products table uses an AutoNumber field called ProductID,and over
to Orders, and I see its called OrderID, and its also an AutoNumber
field.So back to my junction table, and I enter ProductID, select
AutoNumber,then enter OrderID, only this time I select the Number
data type.Im using Number because Access only allows one AutoNumber
field in a table, so I need to use something thats compatible.Now
Ill select both fields, and to do that, you put the cursor in the
gray box next to either field and drag.And with both fields
selected, Ill click Primary Key here on the ribbon.We need both
fields to ensure that each order, and everything in each order, is
uniquely identified.So Ill use Ctrl + S to save the table, and call
it Order Details.So now we can create the relationships.Start by
closing all your open tables, and then select Database Tools, and
Relationships.Select Show Table, here on the Design tab, add the
Order Details, Orders, and Products tables, and close the
dialog.Arrange the tables so Order Details is in the middle,and
then Ill drag the OrderID field from Orders over to the junction
table.In the Edit Relationships dialog, select Enforce Referential
Integrity and create the relationship.Do the same with the
ProductID field, enforce referential integrity, choose Create, and
thats it.Youve just created a many-to-many relationship.So next,
Ill show you how to create a one-to-one relationship, so keep
going.
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Lets create a one-to-one relationship. You wont do this very
often, but you need to know how.And for this video, Ive added two
tables to the database, one for a set of company cars,and another
for the employees who drive them.You can see the company uses one
type of car, so the way to identify who drives what is to use the
license number field.Now for this video, were going to pretend that
each employee is assigned to a single car,but before I start, you
need to know another new term: Index.Access indexes the fields in
your databases, and it uses them like you use the index in a bookto
find data quickly.Indexes are on by default, but you can change
their behavior or turn them off.Thats important, because you create
a one-to-one relationship by first creating a one-to-many
relationshipand then changing the indexes for the key fields in
both tables.So Ill start by creating the relationship, and this
time Ill use the Lookup Wizard.Ill get values from another table,
and thats the Company Cars table, I want the license number
field,and I wont bother with a sort order because Im dealing with a
small number of values.Ill hide the key column, call the field
Company Car, and I wont enable data integrity.This is probably the
only time you wont turn referential integrity on.That way, if I
delete an employee, I dont accidentally delete a car.Click Finish,
and now I have a foreign key, and an easy way to pick license
numbers.So next, make sure you have the field selected, unlike me,
then select Fields, then Indexed, and Unique.Now go to the Company
Cars table and select the primary key, if it isnt already.You can
see its also indexed and unique because its an AutoNumber field, so
were done.If youre in Design view, you can see the Indexed property
down here in the Field Properties pane.And if you open the list for
the property, you see three values.You can use two of these values
in a one-to-one relationship, either No, or Yes, No Duplicates.And,
if we start the Relationships pane and look at the relationship,you
can see its just a plain line with no 1 or infinity symbol.So now
our database has all three types of relationships.Our tables are
linked together, but more than that, weve also made it possible to
write queries.Your table relationships have a big impact on query
design, and well see how that works in the next course, so keep
going.
Lookup WizardUse the
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Course summaryCreate table relationships (Access basics, part
2)
Lookup WizardUse the
paneMany-to-manyOne-to-one3:191:313:253:065:07Intro & typesWhy
relationshipsThey link your tables, so theyre a critical part of
your database, and they inform your queries.Relationship
typesOne-to-one, one-to-many, and many-to-many. One-to-many is most
common, many-to-many are part of most transaction and student
databases, 1:1 is rare.Create a one-to-manyIn datasheet view,
select your foreign key or a blank field.Foreign key: Select the
field, then Fields > More Fields > Lookup & Relationship,
and follow the wizard. Blank field: Select the header, then Lookup
& Relationship and follow the wizard. Use the Relationships
paneChild table must have a foreign key. Select Database Tools >
Relationships, then drag the primary key from the parent table to
the foreign key in the child table. Use Edit Relationships to set
referential integrity, and cascading updates/deletes as
needed.Many-to-manyCreate > Table Design, create a junction
table. In that table, duplicate the primary keys from the tables on
both sides of the relationship, select both fields and set them as
a composite key. One-to-oneCreate a one-to-many relationship, then:
Design view: Set the Indexed property for both keys to No, or Yes,
No Duplicates.Ribbon, set both keys to Indexed and Unique.
See alsoGuide to table relationshipsCreate a relationshipEdit a
relationshipDelete a relationshipMore training coursesOffice
Compatibility Pack
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