text: -- 测试NOT NULL
  CREATE TABLE test_not_null(
      a varchar(20),
      b varchar(20) not null
  );
  INSERT test_not_null(a,b) VALUES ('','');
  INSERT test_not_null(a,b) VALUES (NULL,'1');
  INSERT test_not_null(a,b) VALUES (NULL,NULL);
  
  -- 测试默认值DEFAULT
  CREATE TABLE test_default(
      id INT UNSIGNED AUTO_INCREMENT KEY,
      username VARCHAR(20) NOT NULL,
      age TINYINT UNSIGNED DEFAULT 18,
      email VARCHAR(50) NOT NULL DEFAULT '11@qq.com'
  );
  INSERT test_default(username) VALUES ('kings');
  INSERT test_default(username,age,email) VALUES ('kings',30,'imooc@qq.com');
  
  -- 测试枚举ENUM
  CREATE TABLE test_enum1(
      id INT UNSIGNED AUTO_INCREMENT KEY,
      sex ENUM('a','b','c') DEFAULT('a')
  );
  
  -- 测试UNIQUE KEY
  CREATE TABLE test_unique(
      id INT UNSIGNED AUTO_INCREMENT KEY,
      usernmae VARCHAR(20) NOT NULL UNIQUE KEY,
      card VARCHAR(10) UNIQUE KEY
  );
  INSERT test_unique(usernmae,card) VALUES('kings',NULL);
  
  
  
  