Establishing relationships between tables enables you to link tables in a query.
If you have a problem with a computer and need to contact the manufacturer, you can look up the computer's record in the Computer
table and, with the ManufId value in the record, find the manufacturer's contact information.
Another benefit to establishing relationships between tables is that, if two tables are in a
one-to-many relationship, you can ensure that every record in the table on the many side has a related record in the table on the
one side.
In the graphic above, it would make no sense to enter a manufacturer into a record describing a computer unless that manufacturer was
listed in the Manufacturers table.
| Lesson 5 | Table Column Rules |
| Objective | List the rules for table columns. |
When you create a table, you use the columns to store attributes of the entity represented by the table.
For example, a computer might have attributes of manufacturer, processor speed, amount of random access memory, hard-disk space, floppy-disk drive, speed of a CD-ROM drive, and monitor size.
Each of those attributes is represented as a column in the Computer table.
For example, a computer might have attributes of manufacturer, processor speed, amount of random access memory, hard-disk space, floppy-disk drive, speed of a CD-ROM drive, and monitor size.
Each of those attributes is represented as a column in the Computer table.
-
The column must have a unique name within the table.
-
The values for the column must come from a single domain .
(You will learn about column domains in a later lesson.)
The first rule seems obvious: every column must represent a different attribute of the entity represented by the table. If the same
attribute occurs in more than one table, a relationship exists between those two tables. Imagine that your database contains two
tables: one listing computer manufacturers, the other listing computers owned by your company.
- Domain: Determines the type of data values that are permitted for that attribute.
- relationship: If the same attribute occurs in more than one table, a relationship exists between those two tables.
If you enforce referential integrity between two tables, your Relational Database Management System (RDBMS) will prevent
you from making that mistake.
one-to-many relationship :In relational database design, a one-to-many (1:N) relationship exists when, for one instance of entity A, there exists zero, one, or many instances of entity B; but for one instance of entity B, there exists zero or one instance of entity A.
one-to-many relationship :In relational database design, a one-to-many (1:N) relationship exists when, for one instance of entity A, there exists zero, one, or many instances of entity B; but for one instance of entity B, there exists zero or one instance of entity A.
-
Referential integrity: The means of maintaining the integrity of data between one or more tables that relate to each other. In other
words, a column of data in a table has a null or matching value in a corresponding table.
Referential integrity is usually enforced with foreign keys. - Relational Database Management System (RDBMS) :A software package that manages and provides access to a database. These packages follow Codd’s 12 rules of relational databases and normally use SQL to access data.
