CPSC-310 Database Systems Professor Jianer Chen Room 315C HRBB Lecture #3 Source: modification of slides by Jeffrey Ullman of Stanford U.

Outline of Course Representing things by tables E-R model (Ch. 4) Good table structures Functional Dependencies (Ch. 3) Basic operations on relations Relational Algebra (Ch. 2) Storage management (Chs. 13-14) SQL languages in DDL/DML (Ch. 6) Query processing (Chs. 15-16) More on SQL (Chs. 7-9) Transition processing (Chs. 17-19)

Outline of Course Representing things by tables E-R model (Ch. 4) Good table structures Functional Dependencies (Ch. 3) Basic operations on relations Relational Algebra (Ch. 2) Storage management (Chs. 13-14) SQL languages in DDL/DML (Ch. 6) Query processing (Chs. 15-16) More on SQL (Chs. 7-9) Transition processing (Chs. 17-19)

Roles Sometimes an entity set appears more than once in a relationship.

Roles Sometimes an entity set appears more than once in a relationship. Example 1. Married relationship Husband Wife Bob Ann Joe Sue … … Drinkers Married

Roles Sometimes an entity set appears more than once in a relationship. Example 1. Married relationship Example 2. Buddies relationship Buddy1 Buddy2 Bob Ann Joe Sue Ann Bob Joe Moe … … Husband Wife Bob Ann Joe Sue … … Drinkers Married Drinkers Buddies

Roles Sometimes an entity set appears more than once in a relationship. Example 1. Married relationship Example 2. Buddies relationship Label the edges between the relationship and the entity set with names called roles. Buddy1 Buddy2 Bob Ann Joe Sue Ann Bob Joe Moe … … Husband Wife Bob Ann Joe Sue … … Drinkers Married Drinkers Buddies

Roles Sometimes an entity set appears more than once in a relationship. Example 1. Married relationship Example 2. Buddies relationship Label the edges between the relationship and the entity set with names called roles. Buddy1 Buddy2 Bob Ann Joe Sue Ann Bob Joe Moe … … Husband Wife Bob Ann Joe Sue … … Drinkers Married husband wife Drinkers Buddies

Roles Sometimes an entity set appears more than once in a relationship. Example 1. Married relationship Example 2. Buddies relationship Label the edges between the relationship and the entity set with names called roles. Buddy1 Buddy2 Bob Ann Joe Sue Ann Bob Joe Moe … … Husband Wife Bob Ann Joe Sue … … Drinkers Married husband wife Drinkers Buddies 1 2

Subclasses Subclass = special case = fewer entities = more properties.

Subclasses Subclass = special case = fewer entities = more properties. Assume subclasses form a tree, i.e., no multiple inheritance.

Subclasses Subclass = special case = fewer entities = more properties. Assume subclasses form a tree, i.e., no multiple inheritance. Isa triangles indicate the subclass relationship, which points to the superclass.

Subclasses Subclass = special case = fewer entities = more properties. Assume subclasses form a tree, i.e., no multiple inheritance. Isa triangles indicate the subclass relationship, which points to the superclass. Example. Ales are a kind of beer. Not every beer is an ale, but some are. Suppose that in addition to all the properties (attributes and relationships) of beers, ales also have the attribute color.

Subclasses Subclass = special case = fewer entities = more properties. Assume subclasses form a tree, i.e., no multiple inheritance. Isa triangles indicate the subclass relationship, which points to the superclass. Example. Ales are a kind of beer. Not every beer is an ale, but some are. Suppose that in addition to all the properties (attributes and relationships) of beers, ales also have the attribute color. name Beers manf isa Ales color

Keys A key is a set of attributes for an entity set such that no two entities in this set agree on all the attributes of the key.

Keys A key is a set of attributes for an entity set such that no two entities in this set agree on all the attributes of the key. It is allowed for two entities to agree on some, but not all, of the key attributes.

Keys A key is a set of attributes for an entity set such that no two entities in this set agree on all the attributes of the key. It is allowed for two entities to agree on some, but not all, of the key attributes. We must designate a key for each entity set.

Keys A key is a set of attributes for an entity set such that no two entities in this set agree on all the attributes of the key. It is allowed for two entities to agree on some, but not all, of the key attributes. We must designate a key for each entity set. In an E/R diagram, underline the key attribute(s).

Keys A key is a set of attributes for an entity set such that no two entities in this set agree on all the attributes of the key. It is allowed for two entities to agree on some, but not all, of the key attributes. We must designate a key for each entity set. In an E/R diagram, underline the key attribute(s). Courses dept number hours room

Keys A key is a set of attributes for an entity set such that no two entities in this set agree on all the attributes of the key. It is allowed for two entities to agree on some, but not all, of the key attributes. We must designate a key for each entity set. In an E/R diagram, underline the key attribute(s). Courses dept number hours room Note that hours and room could also serve as a key, but we must select only one key.

Keys A key is a set of attributes for an entity set such that no two entities in this set agree on all the attributes of the key. It is allowed for two entities to agree on some, but not all, of the key attributes. We must designate a key for each entity set. In an E/R diagram, underline the key attribute(s). In an Isa hierarchy, only the root entity set has a key, which is the key for all entities in the hierarchy. Courses dept number hours room Note that hours and room could also serve as a key, but we must select only one key.

Keys A key is a set of attributes for an entity set such that no two entities in this set agree on all the attributes of the key. It is allowed for two entities to agree on some, but not all, of the key attributes. We must designate a key for each entity set. In an E/R diagram, underline the key attribute(s). In an Isa hierarchy, only the root entity set has a key, which is the key for all entities in the hierarchy. Beers Ales isa name manf color Courses dept number hours room Note that hours and room could also serve as a key, but we must select only one key.

Weak Entity Sets Occasionally, entities of an entity set need “help” to identify them uniquely.

Weak Entity Sets Occasionally, entities of an entity set need “help” to identify them uniquely. Entity set E is said to be weak if in order to identify entities of E uniquely, we need to follow one or more many-one relationships from E and include the key of the related entities from the connected entity sets.

Example (player) name is almost a key for football players, but there might be two players with the same name.

Example (player) name is almost a key for football players, but there might be two players with the same name. (player) number is certainly not a key, since players on two teams could have the same number.

Example (player) name is almost a key for football players, but there might be two players with the same name. (player) number is certainly not a key, since players on two teams could have the same number. But number, together with the (team) name related to the player by relationship Plays-on should be unique.

Example (player) name is almost a key for football players, but there might be two players with the same name. (player) number is certainly not a key, since players on two teams could have the same number. But number, together with the (team) name related to the player by relationship Plays-on should be unique. Players Teams Plays-on name number

Example (player) name is almost a key for football players, but there might be two players with the same name. (player) number is certainly not a key, since players on two teams could have the same number. But number, together with the (team) name related to the player by relationship Plays-on should be unique. Players Teams Plays-on name number Double diamond for supporting many-one relationship. Double rectangle for the weak entity set.

Weak Entity-Set Rules A weak entity set has one or more many-one relationships to other (supporting) entity sets.

Weak Entity-Set Rules A weak entity set has one or more many-one relationships to other (supporting) entity sets. Not every many-one relationship from a weak entity set need be supporting.

Weak Entity-Set Rules A weak entity set has one or more many-one relationships to other (supporting) entity sets. Not every many-one relationship from a weak entity set need be supporting. The key for a weak entity set is its own underlined attributes and the keys for the supporting entity sets.

Weak Entity-Set Rules A weak entity set has one or more many-one relationships to other (supporting) entity sets. Not every many-one relationship from a weak entity set need be supporting. The key for a weak entity set is its own underlined attributes and the keys for the supporting entity sets. Example. (player) number and (team) name is a key for Players in the previous example.

Weak Entity-Set Rules A weak entity set has one or more many-one relationships to other (supporting) entity sets. Not every many-one relationship from a weak entity set need be supporting. The key for a weak entity set is its own underlined attributes and the keys for the supporting entity sets. Example. (player) number and (team) name is a key for Players in the previous example. Players Teams Plays-on name number

How to make a good E/R diagram?

How to make a good E/R diagram? Avoid redundancy.

How to make a good E/R diagram? Avoid redundancy. Don’t use an entity set when an attribute will do.

How to make a good E/R diagram? Avoid redundancy. Don’t use an entity set when an attribute will do. Limit the use of weak entity sets.

How to make a good E/R diagram? Avoid redundancy. Don’t use an entity set when an attribute will do. Limit the use of weak entity sets.

Avoiding Redundancy Redundancy occurs when we say the same thing in two or more different ways.

Avoiding Redundancy Redundancy occurs when we say the same thing in two or more different ways. Redundancy wastes space

Avoiding Redundancy Redundancy occurs when we say the same thing in two or more different ways. Redundancy wastes space and (more importantly) encourages inconsistency.

Avoiding Redundancy Redundancy occurs when we say the same thing in two or more different ways. Redundancy wastes space and (more importantly) encourages inconsistency. The two instances of the same fact may become inconsistent if we change one and forget to change the other (update anomaly), or when we delete instances (deletion anomaly).

Avoiding Redundancy Redundancy occurs when we say the same thing in two or more different ways. Redundancy wastes space and (more importantly) encourages inconsistency. The two instances of the same fact may become inconsistent if we change one and forget to change the other (update anomaly), or when we delete instances (deletion anomaly). Beers name manf manfAddr Bad: This design repeats the manufacturer’s address once for each beer and loses the address if there are temporarily no beers for a manufacturer.

Avoiding Redundancy Redundancy occurs when we say the same thing in two or more different ways. Redundancy wastes space and (more importantly) encourages inconsistency. The two instances of the same fact may become inconsistent if we change one and forget to change the other (update anomaly), or when we delete instances (deletion anomaly). Beers name manf manfAddr name name addr ManfBy Beers Manfs Bad: This design repeats the manufacturer’s address once for each beer and loses the address if there are temporarily no beers for a manufacturer. Good: This design gives the address of each manufacturer exactly once.

Avoiding Redundancy Redundancy occurs when we say the same thing in two or more different ways. Redundancy wastes space and (more importantly) encourages inconsistency. The two instances of the same fact may become inconsistent if we change one and forget to change the other (update anomaly). Beers Manfs ManfBy name manf addr Another example of bad designs 46

How to make a good E/R diagram? Avoid redundancy. Don’t use an entity set when an attribute will do. Limit the use of weak entity sets.

Entity Sets Versus Attributes An entity set should satisfy at least one of the following conditions:

Entity Sets Versus Attributes An entity set should satisfy at least one of the following conditions: It is more than the name of something; it has at least one nonkey attribute,

Entity Sets Versus Attributes An entity set should satisfy at least one of the following conditions: It is more than the name of something; it has at least one nonkey attribute, or It is the “many” in a many-one or many-many relationship.

Entity Sets Versus Attributes An entity set should satisfy at least one of the following conditions: It is more than the name of something; it has at least one nonkey attribute, or It is the “many” in a many-one or many-many relationship. Beers Manfs ManfBy name Bad: The manufacturer is nothing but a name, and is not at the “many” end of any relationship, it should not be an entity set.

Entity Sets Versus Attributes An entity set should satisfy at least one of the following conditions: It is more than the name of something; it has at least one nonkey attribute, or It is the “many” in a many-one or many-many relationship. Beers Manfs ManfBy name Beers name manf Good: Since nothing is recorded for manufacturers besides their names, there is no need to make the manufacturer an entity set. Bad: The manufacturer is nothing but a name, and is not at the “many” end of any relationship, it should not be an entity set.

Entity Sets Versus Attributes An entity set should satisfy at least one of the following conditions: It is more than the name of something; it has at least one nonkey attribute, or It is the “many” in a many-one or many-many relationship. Beers Manfs ManfBy name Beers name manf Good: However, if we also need to record manufacturers’ addresses: Manfs deserves to be an entity set because of the nonkey attribute addr. Beers deserves to be an entity set because it is the “many” of the many-one relationship ManfBy.

Entity Sets Versus Attributes An entity set should satisfy at least one of the following conditions: It is more than the name of something; it has at least one nonkey attribute, or It is the “many” in a many-one or many-many relationship. Beers Manfs ManfBy name addr Good: However, if we also need to record manufacturers’ addresses: Manfs deserves to be an entity set because of the nonkey attribute addr. Beers deserves to be an entity set because it is the “many” of the many-one relationship ManfBy.

Entity Sets Versus Attributes An entity set should satisfy at least one of the following conditions: It is more than the name of something; it has at least one nonkey attribute, or It is the “many” in a many-one or many-many relationship. Beers Manfs ManfBy name addr Good: However, if we also need to record manufacturers’ addresses: Manfs deserves to be an entity set because of the nonkey attribute addr. Beers deserves to be an entity set because it is the “many” of the many-one relationship ManfBy.

Entity Sets Versus Attributes An entity set should satisfy at least one of the following conditions: It is more than the name of something; it has at least one nonkey attribute, or It is the “many” in a many-one or many-many relationship. Beers Manfs ManfBy name addr Good: However, if we also need to record manufacturers’ addresses: Manfs deserves to be an entity set because of the nonkey attribute addr. Beers deserves to be an entity set because it is the “many” of the many-one relationship ManfBy.

How to make a good E/R diagram? Avoid redundancy. Don’t use an entity set when an attribute will do. Limit the use of weak entity sets.

Don’t Overuse Weak Entity Sets Beginning database designers often doubt that anything could be a key by itself. They make all entity sets weak, supported by all other entity sets to which they are linked.

Don’t Overuse Weak Entity Sets Beginning database designers often doubt that anything could be a key by itself. They make all entity sets weak, supported by all other entity sets to which they are linked. In reality, we usually create unique ID’s for entity sets. Examples include social-security numbers, automobile VIN’s etc.

When Do We Need Weak Entity Sets? The usual reason is that there is no global authority capable of creating unique ID’s.

When Do We Need Weak Entity Sets? The usual reason is that there is no global authority capable of creating unique ID’s. Example: it is unlikely that there could be an agreement to assign unique player numbers across all football teams in the world.

Outline of Course Representing things by tables E-R model (Ch. 4) Good table structures Functional Dependencies (Ch. 3) Basic operations on relations Relational Algebra (Ch. 2) Storage management (Chs. 13-14) SQL languages in DDL/DML (Ch. 6) Query processing (Chs. 15-16) More on SQL (Chs. 7-9) Transition processing (Chs. 17-19)

Outline of Course Representing things by tables E-R model (Ch. 4) Good table structures Functional Dependencies (Ch. 3) Basic operations on relations Relational Algebra (Ch. 2) Storage management (Chs. 13-14) SQL languages in DDL/DML (Ch. 6) Query processing (Chs. 15-16) More on SQL (Chs. 7-9) Transition processing (Chs. 17-19)

From E/R Diagrams to Relations (Converting E/R diagrams into tables)

From E/R Diagrams to Relations (Converting E/R diagrams into tables) a relation is a table

From E/R Diagrams to Relations (Converting E/R diagrams into tables) a relation is a table name manf Winterbrew Pete’s Bud Lite Anheuser-Busch Beers

From E/R Diagrams to Relations (Converting E/R diagrams into tables) a relation is a table name manf Winterbrew Pete’s Bud Lite Anheuser-Busch Beers table/relation name

From E/R Diagrams to Relations (Converting E/R diagrams into tables) a relation is a table Attributes (column headers) name manf Winterbrew Pete’s Bud Lite Anheuser-Busch Beers table/relation name

From E/R Diagrams to Relations (Converting E/R diagrams into tables) a relation is a table Attributes (column headers) name manf Winterbrew Pete’s Bud Lite Anheuser-Busch Beers Tuples (rows) table/relation name

From E/R Diagrams to Relations (Converting E/R diagrams into tables) Schemas

From E/R Diagrams to Relations (Converting E/R diagrams into tables) Schemas Relation schema = relation name and attribute list.

From E/R Diagrams to Relations (Converting E/R diagrams into tables) Schemas Relation schema = relation name and attribute list. Optionally: types of attributes

From E/R Diagrams to Relations (Converting E/R diagrams into tables) Schemas Relation schema = relation name and attribute list. Optionally: types of attributes Example: Beers(name, manf) or Beers(name: string, manf: string)

From E/R Diagrams to Relations (Converting E/R diagrams into tables) Schemas Relation schema = relation name and attribute list. Optionally: types of attributes Example: Beers(name, manf) or Beers(name: string, manf: string) Database = collection of relations.

From E/R Diagrams to Relations (Converting E/R diagrams into tables) Schemas Relation schema = relation name and attribute list. Optionally: types of attributes Example: Beers(name, manf) or Beers(name: string, manf: string) Database = collection of relations. Database schema = all relation schemas in the database.

From E/R Diagrams to Relations (Converting E/R diagrams into tables) Why Relations?

From E/R Diagrams to Relations (Converting E/R diagrams into tables) Why Relations? Very simple model. Often matches how we think about data. Abstract model that underlies SQL, the most important database language today.

From E/R Diagrams to Relations (Converting E/R diagrams into tables)

From E/R Diagrams to Relations (Converting E/R diagrams into tables) an entity set  a relation (attributes  attributes).

From E/R Diagrams to Relations (Converting E/R diagrams into tables) an entity set  a relation (attributes  attributes). Beers name manf Relation: Beers(name, manf)

From E/R Diagrams to Relations (Converting E/R diagrams into tables) an entity set  a relation (attributes  attributes). a relationship  a relation with attributes: * keys of the connected entity sets. * attributes of the relationship itself.

From E/R Diagrams to Relations (Converting E/R diagrams into tables) an entity set  a relation (attributes  attributes). a relationship  a relation with attributes: * keys of the connected entity sets. * attributes of the relationship itself. name addr name manf Drinkers Beers

From E/R Diagrams to Relations (Converting E/R diagrams into tables) an entity set  a relation (attributes  attributes). a relationship  a relation with attributes: * keys of the connected entity sets. * attributes of the relationship itself. name addr name manf Drinkers Likes Beers

From E/R Diagrams to Relations (Converting E/R diagrams into tables) an entity set  a relation (attributes  attributes). a relationship  a relation with attributes: * keys of the connected entity sets. * attributes of the relationship itself. name addr name manf Drinkers Likes Beers Likes(drinker, beer)

From E/R Diagrams to Relations (Converting E/R diagrams into tables) an entity set  a relation (attributes  attributes). a relationship  a relation with attributes: * keys of the connected entity sets. * attributes of the relationship itself. Likes(drinker, beer) Favorite(drinker, beer) Drinkers Beers Likes Favorite name addr manf

From E/R Diagrams to Relations (Converting E/R diagrams into tables) an entity set  a relation (attributes  attributes). a relationship  a relation with attributes: * keys of the connected entity sets. * attributes of the relationship itself. name addr name manf Drinkers Likes Beers 2 1 Favorite Likes(drinker, beer) Buddies Favorite(drinker, beer) Buddies(name1, name2)

From E/R Diagrams to Relations (Converting E/R diagrams into tables) an entity set  a relation (attributes  attributes). a relationship  a relation with attributes: * keys of the connected entity sets. * attributes of the relationship itself. name addr name manf Drinkers Likes Beers 2 1 Favorite Likes(drinker, beer) Buddies Favorite(drinker, beer) husband wife Buddies(name1, name2) Married Married(husband, wife)

From E/R Diagrams to Relations (Converting E/R diagrams into tables) an entity set  a relation (attributes  attributes). a relationship  a relation with attributes: * keys of the connected entity sets. * attributes of the relationship itself. Likes(drinker, beer) Favorite(drinker, beer) Married(husband, wife) Buddies(name1, name2) Drinkers Beers Likes Favorite Married husband wife name addr manf Buddies 1 2