Codd felt MIS productivity from using relational modeling would come from three objectives. What are the objectives? Briefly explain each one.
The first objective, data independence, delineates a boundary between the logical and physical aspects of database management. The second objective, communicability, promotes effective and efficient communications between users and MIS personnel. It gives the users and the MIS personnel a simple language to use to communicate. The third objective, set processing, allows the database analyst to process records a set at time, instead of a record at a time.
What do you call a key that is the primary key of one table and an attribute in another table? Hint: this key is often used to join two tables together.
A foreign key
When a retrieval language can express all relational algebra operators, it is considered what?
Relationally complete.
4.
Most of the eight operators in relational algebra require two relations. Two operators only require a single relation. Which ones are they and what do they do?
Restrict and project are the two operators that require only one operator. Restrict extracts specified rows from a single relation. See figure 9-2 for an illustration of this command. Project extracts specified columns from a table. See figure 9-3 for an illustration of this command.
5.
Identify and describe two of Codd’s 12 rules.
Rule 1: The information rule--All data must appear to be stored as values in a table.
Rule 2: Guaranteed access--All values in a database must be addressable by specifying a table’s name, column name, and the primary key of the row in which it is stored.
Rule 3: Systematic treatment of null values-- There must be a distinct representation for all unknown or inappropriate data.
Rule 4: Active on-line catalog of the relational model--There must be an on-line catalog that describes the relational model and which authorized users can query using a standard query language.
Rule 5: Comprehensive data sublanguage rule--A relational language must be able to support data definition, data manipulation, security and integrity constraints, and transaction processing operations. It must also support both interactive querying and application programming.
Rule 6: View updating rule--All views that are theoretically updateable must also be updateable using the DBMS.
Rule 7: High level insert, update, and delete--The system must support set-at-a-time operations.
Rule 8: Physical data independence--Applications programs must be able to run without changes even if a different storage device is used or an index is created.
Rule 9: Logical data independence--Application programs should not be affected when information that preserves changes to base tables changes. That is, if a column is added to a table or a new table is added to a database, it does not impact the application programs.
Rule 10: Integrity independence--Integrity constraints must be part of a database’s definition, not part of its application programs.
Rule 11: Distribution independence -The introduction of a distributed DBMS or redistribution of the existing data should have no impact on existing applications.
Rule 12: Nonsubversion rule--It should not be possible to subvert security or integrity by using a record-at-a-time interface (i.e., using a COBOL program with embedded SQL commands to bypass security).
6.
If a primary key is composed of three attributes (i.e. three separate fields), can one of them be null as long as the other two are not null?
No, this violates the entity integrity rule. If any component of the primary key is null it implies that the particular entity it describes cannot be defined.
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