Questions and Answers
In the relational data model, a relation is another name for a record.
False
A superkey uniquely identifies a record within a table.
True
Each cell in a table can contain multiple values.
False
Primary key is a candidate key selected to identify records uniquely within a table.
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Order of columns in a relation has significance according to the relational model.
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A domain in the relational model refers to the set of allowable values for all attributes in a relation.
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In a relational database, a Foreign Key column must always match a Primary Key value in the same table.
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Null represents the absence of a value, which is the same as zero or spaces in a database.
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Referential Integrity states that if a Foreign Key exists in a table, its value must match the Primary Key value of some record in another table.
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SQL (Structured Query Language) is the only relational language standardized by ISO.
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Entity Integrity rule dictates that in a base table, every column of a Primary Key can be null.
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Study Notes
Relational Model
- The relational data model describes the world as a collection of relations (or tables).
- A data model is an integrated collection of concepts for describing data, relationships between data, and constraints on the data.
Components of Data Model
- A structural part
- A manipulative part
- A set of integrity rules
RM Terminology
- Relation: a table with columns and rows
- Attribute: a named column of a relation
- Domain: a set of allowable values for one or more attributes
- Tuple: a record of a relation
- Alternative terminology: table, column, record; file, field, row; relation, attribute, tuple
Properties of Relations
- Each table has a distinct name
- Each cell of a table contains exactly one atomic (single) value
- Each column has a distinct name
- Values of a column are all from the same domain
- Each record is distinct; there are no duplicate records
- Order of columns has no significance
- Order of records has no significance, theoretically
Relational Keys
- Superkey: a column, or a set of columns, that uniquely identifies a record within a table
- Candidate Key: a minimal superkey for a relation
- Primary Key: a candidate key selected to identify records uniquely within a table
- Alternate Keys: candidate keys that are not selected to be primary key
- Foreign Key: a column, or set of columns, within one table that matches a candidate key of some (possibly same) table
Relational Integrity
- Null represents a value for a column that is currently unknown or not applicable for a record
- Null deals with incomplete or exceptional data and represents the absence of a value
- Null is not the same as zero or spaces, which are values
Relational Languages
- SQL (Structured Query Language): a standardized language by ISO
- QBE (Query-by-Example): an alternative graphical “point-and-click” way of querying a database
Relational Integrity Rules
- Entity Integrity: in a base table, no column of a primary key can be null
- Referential Integrity: if a foreign key exists in a table, either the foreign key value must match a candidate key value of some record in its home table
- Business Rules: rules that define or constrain some aspect of the business
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