2. DML - Data Manipulation Language

数据操纵语句，用于添加、删除、更新和查询数据库记录，并检查数据完整性，常用的语句关键字主要包括insert、delete、udpate和select等。(增添改查）
事务：A transaction consists of a collection of DML statements that form a logical unit of work.

DML的主要语句如下：

命令	功能
insert	在表中添加新行
update	修改已存在的行
delete	删除已存在的行
truncate	删除表中所有的行
commit	让修改永久生效
savepoint	回滚到保存的一个点
rollback	撤销数据修改
for update	在使用select查询某行数据时，跟这个语句会将行加锁

2.1 Insert

语法结构：

INSERT INTO table [(column [,column ...)]
VALUES			(value [, value ...]);

第一种方式：根据表的列对应名称，直接一一填入对应的数据

09:35:11 SYS@orcl>insert into emp values (7777,'BRUCE','TECH',7839,'30-JUN-85',2500,null,10);1 row created.09:37:59 SYS@orcl>select * from emp;EMPNO ENAME		JOB			    MGR HIREDATE	 SAL	   COMM     DEPTNO
---------- -------------------- -------------------- ---------- --------- ---------- ---------- ----------7369 SMITH		CLERK			   7902 17-DEC-80	 800			207499 ALLEN		SALESMAN		   7698 20-FEB-81	1600	    300 	307521 WARD 		SALESMAN		   7698 22-FEB-81	1250	    500 	307566 JONES		MANAGER 		   7839 02-MAR-81	2975			207654 MARTIN		SALESMAN		   7698 28-SEP-81	1250	   1400 	307698 BLAKE		MANAGER 		   7839 01-MAY-81	2850			307782 CLARK		MANAGER 		   7839 09-JUN-81	2450			107788 SCOTT		ANALIST 		   7566 19-MAR-87	3000			207839 KING 		PRESIDENT			17-NOV-81	5000			107844 TURNER		SALESMAN		   7698 08-SEP-81	1500	      0 	307876 ADAMS		CLERK			   7788 23-MAY-87	1100			207900 JAMES		CLERK			   7698 03-DEC-81	 950			307902 FORD 		ANALYST 		   7566 03-DEC-81	3000			207934 MILLER		CLERK			   7782 23-JAN-82	1300			107777 BRUCE		TECH			   7839 30-JUN-85	2500			1015 rows selected.

第二种方式：直接指明列名然后给值，不需要的列可以直接不用赋值，允许为空的列都可以不用赋值

09:38:09 SYS@orcl>insert into emp (empno,ename,job,sal) values (7979,'LUCY','CLERK',1500);1 row created.09:40:55 SYS@orcl>select * from emp;EMPNO ENAME		JOB			    MGR HIREDATE	 SAL	   COMM     DEPTNO
---------- -------------------- -------------------- ---------- --------- ---------- ---------- ----------7369 SMITH		CLERK			   7902 17-DEC-80	 800			207499 ALLEN		SALESMAN		   7698 20-FEB-81	1600	    300 	307521 WARD 		SALESMAN		   7698 22-FEB-81	1250	    500 	307566 JONES		MANAGER 		   7839 02-MAR-81	2975			207654 MARTIN		SALESMAN		   7698 28-SEP-81	1250	   1400 	307698 BLAKE		MANAGER 		   7839 01-MAY-81	2850			307782 CLARK		MANAGER 		   7839 09-JUN-81	2450			107788 SCOTT		ANALIST 		   7566 19-MAR-87	3000			207839 KING 		PRESIDENT			17-NOV-81	5000			107844 TURNER		SALESMAN		   7698 08-SEP-81	1500	      0 	307876 ADAMS		CLERK			   7788 23-MAY-87	1100			207900 JAMES		CLERK			   7698 03-DEC-81	 950			307902 FORD 		ANALYST 		   7566 0