sqlisnotnull与isnull用法

NULL 值的处理方式与其他值不同。

NULL 用作未知的或不适用的值的占位符。

注释：无法比较 NULL 和 0；它们是不等价的。

is not null实例

5> create table Billings (
6> BankerID INTEGER,
7> BillingNumber INTEGER,
8> BillingDate datetime,
9> BillingTotal INTEGER,
10> TermsID INTEGER,
11> BillingDueDate datetime ,
12> PaymentTotal INTEGER,
13> CreditTotal INTEGER
14>
15> );
16> GO
1>
2> INSERT INTO Billings VALUES (1, 1, '2005-01-22', 165, 1,'2005-04-22',123,321);
3> GO

(1 rows affected)
1> INSERT INTO Billings VALUES (2, 2, '2001-02-21', 165, 1,'2002-02-22',123,321);
2> GO

(1 rows affected)
1> INSERT INTO Billings VALUES (3, 3, '2003-05-02', 165, 1,'2005-04-12',123,321);
2> GO

(1 rows affected)
1> INSERT INTO Billings VALUES (4, 4, '1999-03-12', 165, 1,'2005-04-18',123,321);
2> GO

(1 rows affected)
1> INSERT INTO Billings VALUES (5, 5, '2000-04-23', 165, 1,'2005-04-17',123,321);
2> GO

(1 rows affected)
1> INSERT INTO Billings VALUES (6, 6, '2001-06-14', 165, 1,'2005-04-18',123,321);
2> GO

(1 rows affected)
1> INSERT INTO Billings VALUES (7, 7, '2002-07-15', 165, 1,'2005-04-19',123,321);
2> GO

(1 rows affected)
1> INSERT INTO Billings VALUES (8, 8, '2003-08-16', 165, 1,'2005-04-20',123,321);
2> GO

(1 rows affected)
1> INSERT INTO Billings VALUES (9, 9, '2004-09-17', 165, 1,'2005-04-21',123,321);
2> GO

(1 rows affected)
1> INSERT INTO Billings VALUES (0, 0, '2005-10-18', 165, 1,'2005-04-22',123,321);
2> GO

(1 rows affected)
1>
2>
3> SELECT *
4> FROM Billings
5> WHERE BillingTotal IS NOT NULL
6> GO
BankerID BillingNumber BillingDate BillingTotal TermsID BillingDueDate PaymentTotal CreditTotal
----------- ------------- ----------------------- ------------ ----------- ----------------------- ------------ -----------
1 1 2005-01-22 00:00:00.000 165 1 2005-04-22 00:00:00.000 123 321
2 2 2001-02-21 00:00:00.000 165 1 2002-02-22 00:00:00.000 123 321
3 3 2003-05-02 00:00:00.000 165 1 2005-04-12 00:00:00.000 123 321
4 4 1999-03-12 00:00:00.000 165 1 2005-04-18 00:00:00.000 123 321
5 5 2000-04-23 00:00:00.000 165 1 2005-04-17 00:00:00.000 123 321
6 6 2001-06-14 00:00:00.000 165 1 2005-04-18 00:00:00.000 123 321
7 7 2002-07-15 00:00:00.000 165 1 2005-04-19 00:00:00.000 123 321
8 8 2003-08-16 00:00:00.000 165 1 2005-04-20 00:00:00.000 123 321
9 9 2004-09-17 00:00:00.000 165 1 2005-04-21 00:00:00.000 123 321
0 0 2005-10-18 00:00:00.000 165 1 2005-04-22 00:00:00.000 123 321

(10 rows affected)
1>
2> drop table Billings;
3> GO

is null 判断为空的内容

4>
5> CREATE TABLE titleauthor(
6> au_id varchar(20),
7> title_id varchar(20),
8> au_ord tinyint NULL,
9> royaltyper int NULL
10> )
11> GO
1>
2> insert titleauthor values(null, '2', 1, 60)
3> insert titleauthor values('2', '3', 1, 100)
4> insert titleauthor values('3', '4', 1, 100)
5> insert titleauthor values('4', '5', 1, 100)
6> insert titleauthor values('5', '6', 1, 100)
7> insert titleauthor values('6', '7', 2, 40)
8> insert titleauthor values('7', '8', 1, 100)
9> insert titleauthor values('8', '9', 1, 100)
10> GO

(1 rows affected)

(1 rows affected)

(1 rows affected)

(1 rows affected)

(1 rows affected)

(1 rows affected)

(1 rows affected)

(1 rows affected)
1>
2> * from titleauthor where au_id is null;
3> GO
au_id title_id au_ord royaltyper
-------------------- -------------------- ------ -----------
NULL 2 1 60

(1 rows affected)
1> select * from titleauthor where au_id = null;
2> GO
au_id title_id au_ord royaltyper
-------------------- -------------------- ------ -----------

(0 rows affected)