题目：
The hr user executes the following query on the employees table but does not issue commit, rollback, or any data definition language (DDL) command after that:

SQL> SELECT job
FROM employees
WHERE job='CLERK' FOR UPDATE OF empno;

HR then opens a second session.
Which two operations wait when executed in HR's second session?

A.INSERT INTO employees(empno,ename,job) VALUES (2001,'Harry','CLERK');
B. LOCK TABLE employees IN EXCLUSIVE MODE;
C. SELECT empno,ename FROM employees WHERE job='CLERK';
D. SELECT job FROM employees WHERE job='CLERK' FOR UPDATE OF empno;
E. INSERT INTO employees(empno,ename) VALUES (1289,'Dick');

Answer: BD

【解析】
使用FOR UPDATE子句可以锁定选定的行，以便其他用户在结束事务之前不能锁定或更新该行。只能在顶级SELECT语句中指定此子句，而不能在子查询中指定此子句。
加上OF ... column子句仅锁定联接中特定表或视图的选择行。OF子句中的列仅指示锁定的表或视图行，您指定的特定列并不重要。 但是，必须指定实际的列名，而不是列别名。如果省略此子句，则数据库将锁定查询中所有表中的选定行。
【官方手册】
https://docs.oracle.com/en/database/oracle/oracle-database/12.2/sqlrf/SELECT.html#GUID-CFA006CA-6FF1-4972-821E-6996142A51C6