列出并简要定义安全机制的分类。
Which syntax turns an existing constraint on?（）
A、ALTER TABLE table_name ENABLE constraint_name;
B、ALTER TABLE table_name STATUS = ENABLE CONSTRAINT constraint _ name;
C、ALTER TABLE table_name ENABLE CONSTRAINT constraint _ name;
D、ALTER TABLE table_name TURN ON CONSTRAINT constraint _ name;
You need to create a table named ORDERS that contains four columns: 1.an ORDER_ID column of number data type 2.a CUSTOMER_ID column of number data type 3.an ORDER_STATUS column that contains a character data type 4.a DATE_ORDERED column to contain the date the order was placed When a row is inserted into the table, if no value is provided for the status of the order, the value PENDING should be used instead. Which statement accomplishes this?（）
A、CREATE TABLE orders ( order_id NUMBER(10), customer_id NUMBER(8), order_status NUMBER(10) DEFAULT 'PENDING', date_ordered DATE );
B、CREATE TABLE orders ( order_id NUMBER(10), customer_id NUMBER(8), order_status VARCHAR2(10) = 'PENDING', date_ordered DATE );
C、CREATE OR REPLACE TABLE orders ( order_id NUMBER(10), customer_id NUMBER(8), order_status VARCHAR2(10) DEFAULT 'PENDING', date_ordered DATE );
D、CREATE OR REPLACE TABLE orders ( order_id NUMBER(10), customer_id NUMBER(8), order_status VARCHAR2(10) = 'PENDING', date_ordered DATE );
E、CREATE TABLE orders ( order_id NUMBER(10), customer_id NUMBER(8), order_status VARCHAR2(10) DEFAULT 'PENDING', date_ordered DATE );
F、CREATE TABLE orders ( order_id NUMBER(10), customer_id NUMBER(8), order_status VARCHAR2(10) DEFAULT 'PENDING', date_ordered VARCHAR2 );
Automatic Shared Memory Management is disabled for your database instance. You realize that there arecases of SQL statements performing poorly because of repeated parsing activity， resulting in degradation ofperformance. What would be your next step to improve performance（）
A、Run the SQL Access Advisor
B、Run the Memory Advisor for the sga.
C、Run the Memory Advisor for the pga.
D、Run the Memory Advisor for the shared pool.
E、Run the Memory Advisor for the buffer cache
关于时差的描述，除了哪一个以外都是正确的？（）
A、总时差指的是一个活动不影响项目完工日期的条件下自它的最早开工可以延迟的时间量
B、随着项目的进展和项目计划的变更，时差也随之变化
C、自由时差指的是一个活动不影响任何紧后活动的最早开工的条件下可以延迟的时间量
D、总时差也叫做闲散时间，分享时差或路径时差
当前图层（）被关闭，（）被冻结。
A、不能，可以
B、不能，不能
C、可以，可以
D、可以，不能