sql is not null 与 is null 用法

如果表中的某个列是可选的，那么我们可以在不向该列添加值的情况下插入新纪录或更新已有的记录。这意味着该字段将以 null 值保存。

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)

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(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)