Database Management 7 Flashcards
This rule states that if an attempt is made to delete a record in one table where one or more records with matching foreign key values exist in another table, then the foreign key values are set to NULL so we know that the record they used to point to has been deleted.
Set-to-Null Delete Rule
is an orderly arrangement used to logically access rows in a table; used to locate an item quickly.
Index
Index key is, in effect, the index’s reference point. More formally, an index is an ordered arrangement of keys and pointers. Each key points to the location of the data identified by the key.
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the index key can have only one pointer value (row) associated with it.
unique index
A table can have many indexes, but each index is associated with only one table.
The index key can have one or multiple attributes (i.e. columns).
An index with multiple attributes is called a composite index.
By default, indexes are created by column value in ascending order.
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The Chen notation favors conceptual modeling.
The Crow’s Foot notation favors a more implementation-oriented approach.
The UML notation can be used for both conceptual and implementation modeling.
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in the ERM corresponds to a table—not to a row—in the relational environment. The ERM refers to a table row as an entity instance or entity occurrence. In the Chen, Crow’s Foot, and UML notations, an entity is represented by a rectangle that contains the entity’s name. The entity name, a noun, is usually written in all capital letters.
entity
required attribute is an attribute that must have a value; in other words, it cannot be left empty.
optional attribute is an attribute that does not require a value; therefore, it can be left empty.
domain is the set of possible values for a given attribute.
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uses identifiers—one or more attributes that uniquely identify each entity instance. In the relational model, entities are mapped to tables, and the entity identifier is mapped as the table’s primary key (PK). Identifiers are underlined in the ERD. Key attributes are also underlined in a frequently used shorthand notation for the table structure, called a relational schema.
The ERM
composite identifier, a primary key composed of more than one attribute.
composite attribute : is an attribute that can be further subdivided to yield additional attributes.
single-valued attribute is an attribute that can have only a single value.
Multi-valued attributes are attributes that can have many values.
derived attribute is an attribute whose value is calculated (derived) from other attributes.
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