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空值等价于空字符串、空格或数值0 函数ISNULL(.NULL.)的值与函数EMPTY(.NULL.)的值相同 将一个.NULL.值赋给字段或变量时,不改变该字段或变量原有的数据类型 变量A1的值为.F.,则表达式A1AND.NULL.的值为.NULL.
(′1031′,′曾华′,男,23) (′1031′,′曾华′,NULL,NULL) (NULL,′曾华′,′男′,′23′) (′1031′,NULL,′男′,23)
('1031','曾华',男,23) ('1031','曾华',NULL,NULL) (NULL,'曾华','男','23') ('1031',NULL,'男',23)
null表示不确定的值 null可看作是空白的字符串 null与数值0具有同等的意义 null的默认值为逻辑值.
('1031',‘李林’,男,23) ('1031',‘李林’,NULL) (NULL,‘李林’,‘男’,'23') ('1031',NULL,‘男’,23)
('1031','李林',男,23) ('1031','李林',NULL) (NULL,'李林','男','23') ('1031',NULL,'男',23)
('1031','李林',男,23) ('1031','李林',NULL) (NULL,'李林','男','23') ('1031',NULL,'男',23)
('1031','李林',男,23) ('1031','李林',NULL) (NULL,'李林','男','23') ('1031',NULL,'男',23)
(‘1031’,‘曾华’,男,23) (‘1031’,‘曾华’,NULL,NUL (NULL,‘曾华’,‘男’,‘23’) (‘1031’,‘NULL’,‘男’,23)
Not Null Is Not Null Between 0 and 64 Null
AGE INT) 可以插入到表中的是______。 (‘1031’,‘曾华’,男,23) (‘1031’,‘曾华’,NULL,NULL) (NULL,‘曾华’,‘男’,‘23’) (‘1031’,‘NULL’,‘男’,23)
('1031','曾华',男,23) ('1031','曾华',NULL,NUL (NULL,'曾华','男','23') ('1031',NULL,'男',23)
('1031','李林',男,23) ('1031','李林',NULL) (NULL,'李林','男','23') ('1031',NULL,'男',23)
CHECK PRIMARY KEY NOT NULL UNIQUE
('1031','李林',男,23) ('1031','李林',NULL) (NULL,'李林','男','23') ('1031',NULL,'男',23)
('1031','曾华',男,23) ('1031','曾华',NULL,NULL) (NULL,'曾华','男','23') ('1031',NULL,'男',23)