Intro to ER Diagrams - Introduction to ER Diagrams M otes...

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1 Introduction to ER Diagrams Mote’s Notes 1/31/2011 Entity-Relationship diagrams (or ER diagrams, or simply ERDs) are logical blueprints of relational database models. The crow's foot style of ER diagram is made up of two basic shapes: the rectangle and the connecting line. The crow's foot is not the only style of ER diagram, but it is certainly one of the most popular styles in use today. The rectangle represents a table (aka, entity) of similar data. A table contains one or more fields (or types) of data. A table may contain zero or more rows (or records) of data. Every table and every field must have a name. No two tables may have the same name. No two fields in the same table may have the same name. The rows of a table are not explicitly named. For instance, a database might have a table named Employee. The Employee table might contain fields named SSN, FirstName, LastName, Address, DOB, Salary, etc. The table might contain 10,000 rows… one for each employee. Every table should have a field (or combination of fields) that can be used to uniquely identify the rows of the table. This field(s) is called a primary key. It is denoted by PK in a crow's foot ER diagram. A connecting line represents logical relationships between rows. Frequently, but not always, a relationship is between rows in two different tables. The crow's foot style of ER diagram not only indicates the tables involved in the relationship, but also indicates the relative number of rows involved. These relative numbers are referred to as the
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Intro to ER Diagrams - Introduction to ER Diagrams M otes...

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