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To:Results (Thai) 1:Because all he does not have a home phone, or he is, but we don't know whether (an unknown value) we can have NULL values generally several means, such as an unknown value. The value is orattribute, but none of this tuple (also known as asvalue, which is not yet defined) does not applyto are not examples of the last type of NULL will occur if we add a Visa_status attribute to the student association.Used only with tuples rather than foreign students can devise various codes for other meanings of different NULLvalues.Incorporating of the relational model is the NULL value, the operation (see Chapter 6) has proved difficult, and outside the scope of our presentation. The exact number of NULLvalue in this way, the fare control between math, or compared to other values, for example, a comparison of two NULL values leads to ambiguities — if there is not A customer, and NULLaddresses B, meaning that they are the same. During the design of the database, the best is to avoid most NULLvalues we will discuss this further in chapters 5 and 6 in the context of operations and queries, and chapter 15 in the context of the design of the database and refresh the hit (meaning) of a schema relationship relationships can hit the Aqua.Mapen publishing or type of firm For example, patterns of students ' Association 3.1 figure confirmed that general entity named student Ssn, address, age, and Office_phone Home_phone Gpa each tuple in a relation can be interpreted as the truth, or the confirmation of a specific instance. For example the first tuple in Figure 3.1 confirm the fact that there are students whose name is Benjamin naoe Ssn is 305-61-2435, age 19, and so on. Note that some relationships may also show the facts about an entity. While other relationships may provide the facts about the relationship. Examples of majors plan (Student_ssn, Department_code) has confirmed that students in disciplines that study the relationship. This tuple relationship involves a student discipline matter of his or her own. Relational model refers to the facts about the relationship and the entity regularly when a comparable relationship. This is sometimes due to a decrease in the level of understandability guess relationships represent the type of entity or relationship type. We introduce the entity relationship model (ER) in detail in Chapter 7 describes the concepts of entities and relationships in the details. The mapping process in a different chapter of the creating 9show ER and the (covered in Chapter 8 ER model.) Conceptual data model (see section 3) is converted to a relationship. An alternative interpretation of the relation scheme in this case, the predicate values in each tuple is converted into values that satisfy the predicate. Sample The predicate student (name, Ssn, ...) is actually for five-tuples associated These students picture 3.1. tuples represents the five stages or the facts in the real world, this interpretation is particularly useful in the context of logic programming languages such as Prolog, because relational format allows the use of these languages (see section 26.5) assumption that called the State closed world assumption, only the facts in the universe they present in the extension (the status) of the Association. Any other combination of values to false predicateResults (Thai) 2:Because he did not have a home phone, or that he has, but we don't know if it (the value is unknown) In general we can have several meanings for the value NULL as the value of an unknown value exists, but cannot be used with this tuple does not applyto orattribute (also known as the asvalue has not been defined), for example, in this last category, while An void occurs, if we add a Visa_status attribute to the student association.Used only with tuples as agents of international students it is possible to devise a different code for different means of NULLvalues.Incorporating the difference of NULL values in relational-operation (see Chapter 6) has been proven and is outside the scope of the presentation.At the true meaning of NULLvalue fare control methods that will be included between the mathematical or comparison with other values. For example, a comparison of two NULL values will lead to a confused customer B if both nguai and NULLaddresses have it doesn't mean that they have the same address. During the design of the database, the best is to avoid as much as possible NULLvalues to discuss this further in chapter possible.We 5 and 6 in the context of the operations and commands, and chapter 15 in the context of database design and normalization. The translation of meaning (semantics) of a relation can be interpreted that the relationship include an announcement or confirmation of type. For example, a student on the associated khikwam Figure 3.1 claims in General, Student Affairs named Ssn, address, Home_phone, Office_phone, Age and average grade of each tuple in the relation can be interpreted as a true or an example of a confirmation. For example, in Figure 3.1, first confirm the fact that there are students whose name is Benjamin nabai supplier, Ssn is 305-61-2435, age is 19, and others notice that some relationships might be facts about other agencies, while relationships may be the facts about ...Maphan MAJORS, for example, include relations (Student_ssn, Department_code) claimed that students in academic disciplines.A tuple in this relationship relates to the students of discipline.Hence he or she demonstrates a fact relationship with both agencies and thousands of teacher.Thakan is a relationship which sometimes might SAP the understandability because it has a strong password that will demonstrate the relationship genre juristic Entity or relationships suggested type.We-relationship (ER) model in detail in Chapter 7 that is a legal entity and relationship concepts will be described in the plan one at a time.่ detail.The the steps in chapter 9show how to create different kinds of ER and EER (increased ER model is covered in Chapter 8) conceptual data model (see step 3) has been converted to alternative relationships of interpretation is associated include verb; in this case, the value of each tuple will be interpreted as a response value, such as predicate.For, verb student (name, Ssn, ...) is actually for five-tuples in the relationship of the student.
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