157.View the Exhibit and examine the structure of the ORDERS and CUSTOMERS tables.
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157.View the Exhibit and examine the structure of the ORDERS and CUSTOMERS tables.
Evaluate the following SQL command:
SQL> SELECT o.order_id, c.cust_name, o.order_total, c.credit_limit FROM orders o
JOIN customers c USING (customer_id)
WHERE o.order_total > c.credit_limit
FOR UPDATE ORDER BY o.order_id;
Which two statements are true regarding the outcome of the above query? (Choose two.)
A.It locks all the rows that satisfy the condition in the statement.
B.It locks only the columns that satisfy the condition in both the tables.
C.The locks are released only when a COMMIT or ROLLBACK is issued.
D.The locks are released after a DML statement is executed on the locked rows.
答案:AC
解析:for updae相当于独占锁,锁定符合条件的所有行
A:正确
B:错误,它会锁定整条记录,并不是某一列
C:正确,这里感觉有点问题,说是仅仅当commit和rollbak可以释放,因该是dcl,ddl语句都可以释放的
D:错误,dml语句是不会提交未提交的事务的
我觉得答案因该只有A
Evaluate the following SQL command:
SQL> SELECT o.order_id, c.cust_name, o.order_total, c.credit_limit FROM orders o
JOIN customers c USING (customer_id)
WHERE o.order_total > c.credit_limit
FOR UPDATE ORDER BY o.order_id;
Which two statements are true regarding the outcome of the above query? (Choose two.)
A.It locks all the rows that satisfy the condition in the statement.
B.It locks only the columns that satisfy the condition in both the tables.
C.The locks are released only when a COMMIT or ROLLBACK is issued.
D.The locks are released after a DML statement is executed on the locked rows.
答案:AC
解析:for updae相当于独占锁,锁定符合条件的所有行
A:正确
B:错误,它会锁定整条记录,并不是某一列
C:正确,这里感觉有点问题,说是仅仅当commit和rollbak可以释放,因该是dcl,ddl语句都可以释放的
D:错误,dml语句是不会提交未提交的事务的
我觉得答案因该只有A
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