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22 Cards in this Set
- Front
- Back
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What is the definition of a Relation
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A relation is a named, two-dimensional table of data
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What do tables consists of
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Table consists of rows (records) and columns (attribute or field)
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What are the Requirements for a table to qualify as a relation:
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It must have a unique name
Every attribute value must be atomic (not multivalued, not composite) Every row must be unique (can’t have two rows with exactly the same values for all their fields) Attributes (columns) in tables must have unique names The order of the columns must be irrelevant The order of the rows must be irrelevant |
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What do Relations (tables) correspond with
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Relations (tables) correspond with entity types and with many-to-many relationship types
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The word relation (in relational database) is NOT the same as
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the word relationship (in E-R model)
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Keys are special fields that serve two main purposes:
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Primary keys are unique identifiers of the relation in question. Examples include employee numbers, social security numbers, etc. This is how we can guarantee that all rows are unique
Foreign keys are identifiers that enable a dependent relation (on the many side of a relationship) to refer to its parent relation (on the one side of the relationship) |
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Keys usually are used as
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indexes to speed up the response to user queries
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Keys can be
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simple (a single field) or composite (more than one field)
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A primary key is
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used to uniquely identify a row and a table.
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What are columns in a table
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attributes.
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What are rows in a table
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instances
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What are Integrity Constraints
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Domain Constraints
Allowable values for an attribute. Entity Integrity No primary key attribute may be null. All primary key fields MUST have data Action Assertions Business rules. Recall from previous lecture. |
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What are domain constraints
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only can so many of a certain type
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What are the 3 things when Mapping Regular Entities to Relations
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Simple attributes: E-R attributes map directly onto the relation
Composite attributes: Use only their simple, component attributes Multivalued Attribute–Becomes a separate relation with a foreign key taken from the superior entity |
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What can 2 foreign keys TOGETHER make up
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primary key. Two foreign keys can be a composite to form a primary key.
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what can referential integrity not do?
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Can not delete a row of a parent table as long as there are dependent entities referring to it. Have to delete all the tables that are dependent on the parent table first, if you want to clean up your database.
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Every attribute value is
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atomic
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First Normal Form is used to remove what
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one to many relationships in the table
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Second Normal Form is used to remove what
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many to many relationships in the table
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What are you trying to get in using First normal form and second normal form.
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a one to one relationship in a table.
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What does third normal form do.
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Removes the transitive dependencies
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What are Enterprise Keys
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Primary keys that are unique in the whole database, not just within a single relation
Corresponds with the concept of an object ID in object-oriented systems |
