Is not null and in SQL= Difference between null

Hello everyone , I meet you again , I'm your friend, Quan Jun .

We often meet these two writing methods ：IS NOT NULL And !=NULL. Also often meet the database has qualified !=NULL The data of , But the return is an empty collection . actually , It's because we don't understand the difference between them .

By default , Recommended IS NOT NULL Make conditional judgments , because SQL By default WHERE XX！= Null Your judgment will always return to 0 That's ok , But it won't prompt syntax errors .

Why is that ？

SQL Server In the document for Null The comparison of values defines two rules , If in SQL Server 2000 in ：

Rule one yes yes ANSISQL（SQL-92） Stipulated Null The comparison results of the values are False, Both Null=Null The value is also False. The other is not allowed to follow ANSISQL standard , namely Null=Null by True.：

For example, data sheet test structure ：

Copy code The code is as follows :

ROWNUM     DATA

——————-

1     ‘Liu Yang’

2     Null

3     ‘12345’

Copy

according to ANSI SQL standard , Neither of the following two queries returns any rows ：

Query one : SELECT * FROM test WHERE data=NULL Query two : SELECT * FROM test WHERE data<>NULL

Instead of ANSI SQL standard , Inquire about 1 The second line... Will be returned , Inquire about 2 return 1、3 That's ok .

This is because in the SQL in ,NULL Is a unique data type , It is equivalent to having no value 、 It's an unknown number .NULL And 0、 An empty string 、 The spaces are different . ANSI SQL Standard Null The row of values needs to use the following query ：

Copy code The code is as follows :

  SELECT * FROM test WHERE data IS NULL

Copy

This shows that it is not ANSI SQL In the standard data=NULL Equate to data IS NULL,data<>NULL Equate to data IS NOT NULL.

So we should keep in mind ： By default, the keyword is used for comparison “is null” and “is not null”.

If you must use != null To make conditional judgments , You need to add this command statement ：SET ANSI_NULLS OFF, At this time, the database enters ANSI SQL Non standard mode , You'll find that IS NOT NULL and != null Is equivalent .

The mode switching command is used here SET ANSI_NULLS[ON/OFF].ON Value adoption ANSI SQL Strict standards ,OFF Value is in non-standard compatibility mode . in addition SET ANSI_DEFAULTS [ON/OFF] Command can also realize standard switching , Only this command controls a group that conforms to SQL-92 Standard settings , Among them is Null Value criteria .

By default , Database management program （DB-Library） yes SET ANSI_NULLS by OFF Of . But most of our applications , It's all through ODBC perhaps OLEDB To access the database , As an open and compatible database access program , Maybe it's a compatibility consideration ,SETANSI_NULLS Value is set to ON. Some problems caused by this should be paid attention to . Applications such as stored procedures or custom functions are based on DB-Library Of , By default ,SETANSI_NULLS by OFF, And in such a program , Out of commission SETANSI_NULLS Modify rules in an environment , Only database configuration parameters can be modified .

For example, in the following case ： Your application uses ADODB To access the database , use OleDb perhaps ODBC Data provider . For query one : SELECT * FROM test WHERE data=NULL We can directly send commands to get the query result set , You can also put it in a stored procedure . But the results of the two queries are different . If you use the query command directly , Do not return any rows ; And if you access a stored procedure , Back to page 2 Row data .

Last , We declare once again ： By default, the database , do SQL Use keywords when comparing conditional queries “is null” and “is not null”.

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