对象是由什么表示的？什么事列对象？为什么不能访问表对象和行对象？

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• What does the TRUNCATE statement do?（）

  A、Removes the table

  B、Removes all rows from a table

  C、Shortens the table to 10 rows

  D、Removes all columns from a table

  E、Removes foreign keys from a table

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• The EMP table contains these columns: LAST NAME VARCHAR2(25) SALARY NUMBER(6,2) DEPARTMENT_ID NUMBER(6) You need to display the employees who have not been assigned to any department. You write the SELECT statement: SELECT LAST_NAME, SALARY, DEPARTMENT_ID FROM EMP WHERE DEPARMENT_ID = NULL; What is true about this SQL statement? （）

  A、The SQL statement displays the desired results.

  B、The column in the WHERE clause should be changed to display the desired results.

  C、The operator in the WHERE clause should be changed to display the desired results.

  D、The WHERE clause should be changed to use an outer join to display the desired results.

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• 制图国家标准规定，图纸幅面尺寸应优先选用（）几种基本幅面尺寸。

  A、3

  B、4

  C、5

  D、6

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• 试说明推动分时系统形成和发展的主要动力是什么？

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• 对于二维数组a［1..6，1..8］，设每个元素占2个存储单元，且以列为主序存储，则元素a［4，4］相对于数组空间起始地址的偏移量是（）个存储单元。

  A、28

  B、42

  C、48

  D、54

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