Transcript CS206 --- Electronic Commerce

The Relational Data Model
Tables
Schemas
Conversion from E/R to Relations
Source: slides by Jeffrey Ullman
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A Relation is a Table
Attributes
(column
headers)
Tuples
(rows)
name
Snickers
Twizzlers
manf
M&M/Mars
Hershey
Candies
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Schemas
Relation schema = relation name and
attribute list.
 Optionally: types of attributes.
 Example: Candies(name, manf) or
Candies(name: string, manf: string)
Database = collection of relations.
Database schema = set of all relation
schemas in the database.
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Why Relations?
Very simple model.
Often matches how we think about
data.
Abstract model that underlies SQL, the
most important database language
today.
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From E/R Diagrams to Relations
Entity set -> relation.
 Attributes -> attributes.
Relationships -> relations whose
attributes are only:
 The keys of the connected entity sets.
 Attributes of the relationship itself.
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Entity Set -> Relation
name
manf
Candies
Relation: Candies(name, manf)
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Relationship -> Relation
name
husband
addr
Consumers
1
name
Likes
manf
Candies
2
Buddies
Favorite
wife
Married
Likes(consumer, candy)
Favorite(consumer, candy)
Buddies(name1, name2)
Married(husband, wife)
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Combining Relations
 OK to combine into one relation:
1. The relation for an entity-set E
2. The relations for many-one relationships
of which E is the “many.”
 Example: Consumers(name, addr) and
Favorite(consumer, candy) combine to
make Consumer1(name, addr,
favCandy).
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Risk with Many-Many Relationships
Combining Consumers with Likes would
be a mistake. It leads to redundancy,
as:
name
addr
candy
Sally 123 Maple Twizzler
Sally 123 Maple Kitkat
Redundancy
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Handling Weak Entity Sets
Relation for a weak entity set must
include attributes for its complete key
(including those belonging to other
entity sets), as well as its own, nonkey
attributes.
A supporting relationship is redundant
and yields no relation (unless it has
attributes).
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Example
name
billTo
Logins
name
At
Hosts
location
Hosts(hostName, location)
Logins(loginName, hostName, billTo)
At(loginName, hostName, hostName2)
At becomes part of
Logins
Must be the same
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Subclasses: Three Approaches
1. Object-oriented : One relation per subset of
subclasses, with all relevant attributes.
2. Use nulls : One relation; entities have NULL
in attributes that don’t belong to them.
3. E/R style : One relation for each subclass:
 Key attribute(s).
 Attributes of that subclass.
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Example
name
Candies
manf
isa
color
Chocolates
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Object-Oriented
name
Twizzler
manf
Hershey
Candies
name
Snickers
manf
color
M&M/Mars light
Chocolates
Good for queries like
“find the color of chocolate candies made by M&M/Mars.”
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E/R Style
name
Twizzler
Snickers
manf
Hershey
M&M/Mars
Candies
name
Snickers
color
light
Chocolates
Good for queries like
“find all candies (including chocolates) made
by M&M/Mars.”
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Using Nulls
name
Twizzler
Snickers
manf
Hershey
M&M/Mars
Candies
color
NULL
dark
Saves space unless there are lots
of attributes that are usually NULL.
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