解锁scott:sqlplus / as sysdba;alter user scott account unlock;alter user scott identified by tiger;  /*    DDL操作表        数据库定义语言*/--创建表结构CREATE TABLE STUDENT(    SNO CHAR(10),    SNAME VARCHAR2(20),    AGE NUMBER(3),    SCHOOLDATE DATE,    GRADE NUMBER(1) DEFAULT 1,    CLAZZ NUMBER(2),    EMAIL VARCHAR2(100));--删除表结构DROP TABLE STUDENT;--修改表的名称RENAME BANK TO SXTBANK;-------------------修改表的结构--添加列ALTER TABLE STUDENT ADD(ADDRESS VARCHAR2(100));--删除列ALTER TABLE STUDENT DROP COLUMN ADDRESS;--修改列ALTER TABLE STUDENT MODIFY(EMAIL VARCHAR2(200));/*    DDL约束        规范我们输入的数据                    加入约束有两种时机            创建表的时候            创建表之后                有可能会首先表中数据的干扰    非空:        当前数据项不能为空    唯一:        表中的列的值不能相等但是null除外    主键:        不为空且唯一        一个表中只允许有一个主键        创建主键索引        主键可以同时指定多个列,称为联合主键，但是一般不这么去写    外键    自定义        自定义约束的值*/--测试非空约束INSERT INTO STUDENT(SNAME) VALUES('ZHANGSAN');--添加控制约束(非空和唯一)ALTER TABLE STUDENT MODIFY(EMAIL VARCHAR2(100) NOT NULL);ALTER TABLE STUDENT ADD(ADDRESS VARCHAR2(10) UNIQUE);INSERT INTO STUDENT(SNAME,EMAIL) VALUES('ZHANGSAN','LIYI@BJSXT.COM');INSERT INTO STUDENT(SNAME,EMAIL) VALUES('LISISI','LISISI@BJSXT.COM');INSERT INTO STUDENT(SNAME,EMAIL,ADDRESS) VALUES('LISISI','LISISI@BJSXT.COM','AAA');INSERT INTO STUDENT(SNAME,EMAIL,ADDRESS) VALUES('WANGWUWU','LISISI@BJSXT.COM','AAA');--主键约束ALTER TABLE STUDENT MODIFY(SNO CHAR(10) PRIMARY KEY);CREATE TABLE HELLO(    SNO VARCHAR2(20) PRIMARY KEY);CREATE TABLE HELLO(    SNO VARCHAR2(20),    CONSTRAINT HELL_PK PRIMARY KEY(SNO));--自定义约束CREATE TABLE HI(    AGE NUMBER(3) CHECK(AGE<150));--外键约束CREATE TABLE QQ(    GARDE NUMBER(3) PRIMARY KEY,    GNAME VARCHAR2(20));CREATE TABLE FIREND(    FID NUMBER(3),    NAME VARCHAR2(20),    GRADE NUMBER(3),    FOREIGN KEY(GRADE) REFERENCES QQ(GARDE) ON DELETE CASCADE);--  FOREIGN KEY(GRADE) REFERENCES QQ(GARDE)--  FOREIGN KEY(GRADE) REFERENCES QQ(GARDE) ON DELETE SET NULL--  FOREIGN KEY(GRADE) REFERENCES QQ(GARDE) ON DELETE ----删除数据--(删除的时候要保证没有子记录)--(删除的时候子记录设置为空)--(删除的时候级联删除子记录)INSERT INTO QQ VALUES(1,'高中同学');INSERT INTO QQ VALUES(2,'初中同学');INSERT INTO QQ VALUES(3,'小学同学');INSERT INTO FIREND VALUES(11,'张1丰',1);INSERT INTO FIREND VALUES(22,'张2丰',2);INSERT INTO FIREND VALUES(33,'张3丰',3);INSERT INTO FIREND VALUES(44,'张4丰',1);INSERT INTO FIREND VALUES(55,'张5丰',2);DELETE FROM QQ WHERE GARDE = 1;