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Types of Database Relationships

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There are three different types of database relationships, each named according to the number of table rows that may be involved in the relationship. Each of these three relationship types exists between two tables.

 

One-to-one relationships:- occur when each entry in the first table has one, and only one, counterpart in the second table. One-to-one relationships are rarely used because it is often more efficient to simply put all of the information in a single table.

 

One-to-many relationships:- are the most common type of database relationship. They occur when each record in the first table corresponds to one or more records in the second table but each record in the second table corresponds to only one record in the first table. For example, the relationship between a Teachers table and a Students table in an elementary school database would likely be a one-to-many relationship, because each student has only one teacher, but each teacher may have multiple students.

 

Many-to-many relationships:- occur when each record in the first table corresponds to one or more records in the second table and each record in the second table corresponds to one or more records in the first table. For example, the relationship between a Teachers and a Courses table would likely be many-to-many because each teacher may instruct more than one course and each course may have more than one instructor.

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