comparing null values in sql

The Expert_details modifier (type VARCHAR2 (20), nullable) are retrieved for those records that are empty because the field is of type VARCHAR2 (20), and the SQL statement I use isCopy code code as follows:SELECT * from expert_details twhere t.modifier = ' A record was not retrieved, which does not match the records stored in the table. It was later thought that even empty character storage should be null

Although the person who is proficient in SQL will not have any questions about NULL, but the whole article is still very difficult to find, see an English version, feel good.Tony Hoare invented the null reference in 1965 and considered it a "billions of dollar mistake" he had made. Even today, 50 years later, a null va

this? Everything just because null is a type that represents an "unknown". In other words, it is meaningless to compare null with other values using the regular comparison operator (normal conditional operators). Null is also not equal to NULL (approximate understanding: Th

We often encounter null in the use or maintenance of SQL Server, so what is NULL? The following is a short description from MSDN (see "Null Values"): A value of NULL indicates that's the value is unknown. A value of

Null missing value: If there is null in the expression, the result is also null. The null value is never equal to other values, and it is not equal to itself. Most functions that contain null return

I found that Update DBO. tblinvtransactionSet area_type = 'groups ',Where (area_type = NULL)The result is zero. Then find the following articleChange Update DBO. tblinvtransactionSet area_type = 'group'Where (area_type is null)OK. A problem encountered when I wrote a database query program a few days ago was about the comparison of the null

* From testnull where ID is not null; No rows selected SQL> select * From testnull where id = NULL; No rows selected SQL> select * From testnull where ID! = NULL; No rows selected The Null String ''is displayed as a

, and the MySQL console uses SQL statements to query: SELECT * from qms_branch WHERE Types! = ' dashboard ' The discovery does not include the Performance_trunk branch in the query results. What is the reason for this? Why in the 4th data, the value of the types attribute is not dashboard, but the use of types!= ' dashboard ' will not be able to query the results? Reason Traceability Look at the structure of the data table Qms

Table Sales.orderdetails Order Details Form Sales.orders Order Form Sales.shippers Freight company Table SQL Server Some basic commands: Query Database exists: If db_id ("TestDB") is not null; Check if the table exists: If object_id ("TextDB", "U") is not null, where U represent

a group.Code --drop the unique constraintALTER TABLE [dbo].[test] DROP CONSTRAINT [UQ__test__737584F63A81B327]--insert two null valuesinsert into test values(NULL,12),(NULL,13)select * from testselect Name,SUM(age)as sumage from test group by Name We can see that null is di

In C #, the SQL parameter is passed into the null value and the error solution is displayed, The null in C # is different from the NULL in SQL. The NULL in SQL is represented by C # As

When designing a table, we sometimes argue about whether the field allows null values. The database master Kalen Delaney gave me some suggestions. 1. Never allow null values in the User table 2. the user table definition contains a not null limit. 3. Do not rely on Database

SQL NULL ValueThis chapter explains the is null and is not NULL operators.A NULL value is an unknown data that is missing. By default, the columns of the table can hold NULL values.If a column in the table is optional, we can inse