NOTE 68 Part I EXAM PREPARATION DESIGNING ENTITY
NOTE 68 Part I EXAM PREPARATION DESIGNING ENTITY KEYS . Design entity keys. Considerations include FOREIGN KEY constraints, PRIMARY KEY constraints, and UNIQUE constraints. Specify attributes that uniquely identify records. Specify attributes that reference other entities. Entity key definition is an important part of designing the structure of an ER model. Two types of keys exist: Primary and Foreign. These keys are used in establishing relationships and also provide an easier and more efficient way to retrieve data in the physical implementation. Primary Keys Primary Keys consist of sets of attributes whose values uniquely identify the rows in an entity. Primary Keys give an ID to a row. They make the row unique throughout the entity. This means that rows can easily located by this identifier. Defining Primary Keys Primary Keys Primary Keys can only be used for columns or attributes that don t can be implemented on columns in SQL Server 2000 in three ways. The allow null values. Allowing null values would mean that a row would first is to use the stored procedure not be uniquely identified. Also, the attribute chosen to hold a sp_primarykey. The second way is Primary Key must have values unique throughout the entity. to create a primary key constraint Choosing a Primary Key for a column is relatively simple in contrast on the column. Finally, you can create to choosing entities and attributes. Identifying Primary Keys is also a unique index on a Primary Key part of the Microsoft exam objective, so it s advisable that you have a candidate. good grasp of designing entity keys. A Primary Key should be a numeric value if possible, but this is not a hard and fast rule. To identify a Primary Key, a single investigation has to be taken: whether the column holds true when you ask whether this attribute can carry completely unique, known values throughout the entity. If the answer is a solid yes, no more investigation is needed; the column is worthy of being named a Primary Key column. However, if you notice that none of the attributes in the entity can uniquely identify themselves, it s possible to add an additional surrogate attribute. In many cases surrogate keys are actually a preferred mechanism because the values never need to change. Some designers dislike its use because it adds a column to the data structure that has no
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