4. Relationships One-To-One Each record in one table can have only one matching record in another, and each record in the other can have only one matching record in the first. Not a common relationship. One-To-Many A record in one table can have many matching records in another, but a record in the other has only one matching record in first The most common form of relationship. Many-to-Many A record in one table can have many matching records in another, and a record in the other can have many matching records in the first
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6. ER Diagrams This is a one-to-many relationship - one subject can have many students This is a one-to-one relationship - one student can only have one student ID STUDENT SUBJECT STUDENT STUDENT ID
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8. Creating ER Diagrams Identify attributes Identify entities Year Student ID Surname Student SubjectID Name Subject
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13. Un-normalised Data Repeating groups and Null values A company manufactures and supplies a variety of products to retail suppliers .
![Data Relationships ,[object Object],[object Object]](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)