--創(chuàng)建employees表
CREATE TABLE employees (
  id NUMBER,
  name VARCHAR2(100)  
);
--對(duì)表新增email列，添加約束默認(rèn)空字符串并且非空
ALTER TABLE employees ADD email VARCHAR2(100) DEFAULT ''  NOT NULL;

--創(chuàng)建表employees
CREATE TABLE employees(
id int,   
name VARCHAR(100)   
);
--添加字段email，并添加約束
ALTER TABLE employees ADD email VARCHAR(100) DEFAULT ''  NOT NULL;

--emial字段沒(méi)有添加，默認(rèn)為空字符串，不是null
INSERT INTO employees (id,name) VALUES (1,'TOM');
INSERT INTO employees  VALUES (2,'Carl','');
--email添加一個(gè)值
INSERT INTO employees  VALUES (3,'Jhon','123@qq.com');
--查詢結(jié)果
mysql> SELECT  *  FROM  employees;
+------+------+------------+
| id   | name | email      |
+------+------+------------+
|    1 | TOM  |            |
|    2 | Carl |            |
|    3 | Jhon | 123@qq.com |
+------+------+------------+
3 rows in set (0.00 sec)

ALTER TABLE employees modify email VARCHAR(100);

INSERT INTO employees (id,name) VALUES (1,'TOM');
INSERT INTO employees  VALUES (2,'Carl','');

mysql> SELECT  *  FROM  employees;
+------+------+-------+
| id   | name | email |
+------+------+-------+
|    1 | TOM  | NULL  |
|    2 | Carl |       |
+------+------+-------+
2 rows in set (0.00 sec)

--創(chuàng)建表
CREATE TABLE employees (
id NUMBER,
name VARCHAR2(100)  
);
--新增email字段，添加約束
ALTER TABLE employees ADD email VARCHAR2(100) DEFAULT ''  NOT NULL;

INSERT INTO employees (id,name) VALUES (1,'TOM');
INSERT INTO employees  VALUES (2,'Carl','');

--修改email的約束，改成可以添加空值
ALTER TABLE employees MODIFY email varchar2(100) ;
--插入數(shù)據(jù)
INSERT INTO employees (id,name,email) VALUES (1,'TOM','');  --添加空字符串
INSERT INTO employees  VALUES (2,'Carl',null);