外文翻譯---關(guān)系數(shù)據(jù)庫(kù)的結(jié)構(gòu)-其他專業(yè)(編輯修改稿)
2025-02-24 09:22 本頁(yè)面
【文章內(nèi)容簡(jiǎn)介】 Row:A group of one or more data elements in a database table that describes a person, place, or thing. Column:The ponent of a database table that contains all of the data of the same name and type across all rows. You39。ll learn about database design in the following sections, but let39。s assume for the moment that the majority of the database design is pleted and some tables need to be implemented. Scott creates the EMP table to hold the basic employee information, and it looks something like this: Notice that some fields in the Commission （ COMM） and Manager （ MGR） columns do not contain a value。 they are blank. A relational database can enforce the rule that fields in a column may or may not be empty. In this case, it makes sense for an employee who is not in the Sales department to have a blank Commission field. It also makes sense for the president of the pany to have a blank Manager field, since that employee doesn39。t report to anyone. Field:The smallest piece of information that can be retrieved by the database query language. A field is found at the intersection of a row and a column in a database table. On the other hand, none of the fields in the Employee Number （ EMPNO） column are blank. The pany always wants to assign an employee number to an employee, and that number must be different for each employee. One of the features of a relational database is that it can ensure that a value is entered into this column and that it is unique. The EMPNO column, in this case, is the primary key of the table. Primary Key:A column （ or columns） in a table that makes the row in the table distinguishable from every other row in the same table. Notice the different datatypes that are stored in the EMP table: numeric values, character or alphabetic values, and date values. As you might suspect, the DEPTNO column contains the department number for the employee. But how do you know what department name is associated with what number? Scott created the DEPT table to hold the descriptions for the department codes in the EMP table. The DEPTNO column in the EMP table contains the same values as the DEPTNO column in the DEPT table. In this case, the DEPTNO column in the EMP table is considered a foreign key to the same column in the DEPT table. A foreign key enforces the concept of referential integrity in a relational database. The concept of referential integrity not only prevents an invalid department number from being inserted into the EMP table, but it also prevents a row in the DEPT table from being deleted if there are employees still assigned to that department. Foreign Key:A column （ or columns） in a table that draws its values from a primary or unique key column in another table. A foreign key assists in ensuring the data integrity of a table. Referential IntegrityA method employed by a relational data
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