Relationships between tables are critical in a relational database because they link separated data back together and impact how queries retrieve answers. There are three main relationship types: one-to-many, which is most common; many-to-many, used in transaction and student databases; and one-to-one, which is rare. A primary key uniquely identifies records in a table and cannot be duplicated or contain null values. Foreign keys in one table reference the primary key of another table to represent relationships between them. Relationships are depicted in entity diagrams by drawing lines between matching keys and labeling the "one" side with 1 and the "many" side with ∞.

1. Create Table Relationships

2. Relationships
Why relationships?
They are critical part of your database, because
they link your tables back together after you
separate your data and also because they have a
big impact on how you design your queries and
get answers form your data.

3. Relationship types
Any relational database uses three types of
relationships between its tables:
 One-to-many (Most common)
 Many-to-many (Part of most transaction and
student databases)
 One-to-one (Rare or not most common)

4. Relationship types
One – to - Many
Many– to - Many
One– to - One

5. Primary Key
• A primary key is a field (or combination of fields)
that uniquely identifies each record stored in a
table
• No two records in a table may be exact duplicates
– two records may, however, contain duplicate
data other than the primary key
• Primary key must be unique
• Primary key cannot contain a null value
• Will not change in value
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6. Table Relationships
• A column in a table
that references the
primary key of
another table is
called a foreign key
• Foreign keys and
primary keys
represent data
relationships
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7. Table Relationships
• A relationship is made by matching data in key
fields between two tables
• When creating relationships, remember:
– Related fields may or may not have the same name
but must have the same data type
– When related fields are Number fields, they must
have the same Field Size property setting
– To define a relationship using an AutoNumber primary
key field, the foreign key must be a Number field with
a Field Size property setting of Long Integer
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8. Relationship Types
One-to-one A relationship in which each record in Table A can have only
one matching record in Table B, and vice versa.
One-to-many
Parent-child
A relationship in which a record in Table A can have many
matching records in Table B, but a record in Table B has only
one matching record in Table A. The relationship is
established only if the common field is the primary key in
Table A and the foreign key in Table B.
Many-to-many A relationship in which one record in either Table A or Table
B can relate to many matching records in the other table.
Relational databases cannot directly handle many-to-many
relationships, and these must be replaced by multiple one-
to-many relationships.
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9. Representing table relationships
• In an IE diagram, you
represent relationships by
drawing lines between the
key fields
• The “one” side is indicated
by drawing a 1 beside the
parent key
• The “many” side is
indicated by drawing an
infinity symbol (∞) beside
the foreign key
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