CS669_Lab5HealthInstructions

# CS669_Lab5HealthInstructions - MET CS 669 Database Design...

• Lab Report
• 8
• 100% (3) 3 out of 3 people found this document helpful

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Page 2 of 8 SECTION ONE In this section we will learn to work with subqueries, which significantly extend the expressional power of queries. Through the use of subqueries, a single query can extract result sets that could not be extracted without subqueries. Subqueries enable the query creator to ask the database for many complex structures in a single query. The foundation for learning how subqueries work lies in the expressional nature of the relational model. We learned previously that the operations in the relational model, such as SELECT, PROJECT, and UNION, perform operations on relations and yield a new relation as the result. That is, when one operation operates on a relation, and yields a new relation, we can use a second operation to operate on the result of the first operation. We will take a look at these relational operations in a moment, but first let us look at a simple mathematical example. If we add two plus two to obtain a result of 4, 2 + 2 = 4 , we have applied the plus operation to two numbers. Interestingly, the result of the plus operation is another number. So we can say that the plus operator operates on two numbers, and results in a new number.
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• Fall '15
• Relational model, subqueries, medical procedure

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