Relational Model Stores data as tables –Each column contains values about the same attribute –Each column has a distinct name –Each row contains values for a given entity –Each row is distinct –Order of columns (and rows) is immaterial –Cells are single valued

Terminology

Relationships An association between entities Unary (recursive) Binary –One to Many or Many to Many Ternary Quaternary

Binary Relationships One to one –Each row in a table has at most one matching row in another table One to many Many to many –A row in the first table matches many rows in the second and a row in the second matches many rows in the first

Keys Superkey –an attribute or a set of attributes that uniquely identifies a row within a table Candidate –unique and irreducible identifier of a row, for all time (minimal superkey) Primary –chosen candidate key for a table Alternate

Keys (continued) Secondary –an attribute used for sorting and retrieval Composite –a key made of more than one attribute Foreign –used to designate relationships

Database Design Language (DBDL) Include all attributes (separated by commas) within parentheses with the name of the table outside the parentheses Underline the primary keys List any alternate, secondary and foreign keys identified by the letters AK, SK and FK respectively Foreign keys are followed by an arrow pointing to the relation identified by the foreign key

Representing Relationships One to one –create a foreign key One to many –place primary key of parent in child table Many to many –Break into two one to many relationships

Student # Name Major Student # Course # Grade Course # Name Instructor StudentGradeCourse

Entity Relationship Design Determine potential entities (rectangles) Identify relationships (diamonds) between entities Identify the attributes (ovals) of the entities –(and the attributes for any M:N relationships) Identify the primary keys

Entity-Relationship Diagram First

Symbols for Columns

Integrity rules Entity integrity –No column that is part of the primary key may accept null values. Referential integrity –If table A contains a foreign key that matches the primary key of table B, then values of this foreign key either must match the value of the primary key for the same row in table B or must be null.

Integrity Services Domain integrity –ensures that values assigned to a field are in that field’s domain Relation integrity –ensures that related fields are in the same level of update

Referential Constraints Cascading Updates Option If the value of the primary key in the primary (referenced) table is changed, then all values of the appropriate foreign key in the related (referencing) table are automatically changed to the new value Cascading Deletes Option If you delete a record in the primary (referenced) table, then all records in the related (referencing) table that have matching foreign key values will also be deleted

Main/Subforms First define the 1:N relationship between the tables Create a main form for data from the primary table Create a subform for the data from the related table (Similarly you can display records from the related table as a sub-datasheet in the primary table’s datasheet)

Assignment 2 MS Access 2000 Page AC 4.33 #6-9 (first set up a relationship between the tables)