Usage of Select Null

usage of select null?

Select null is usually used in combination with EXISTS.

eg:- IF EXISTS( select null from ...)

It sets up the Exists status as true if there are records in the select query.
Check this link which has some interesting comments on the usage of select null with Exists: SQL SERVER- IF EXISTS(Select null from table) vs IF EXISTS(Select 1 from table)

Why does Null act differently than “IS NOT NULL”?

From https://dev.mysql.com/doc/refman/8.0/en/problems-with-null.html:

In SQL, the NULL value is never true in comparison to any other value, even NULL. An expression that contains NULL always produces a NULL value unless otherwise indicated in the documentation for the operators and functions involved in the expression.

NULL values represent missing unknown data.

And NULL values are treated differently from other values.

For NULL values with comparison operators, such as =, <, or <> is not possible to test for .

It's rule. So you only can use IS NULL or IS NOT NULL operators instead.

How to ignore NULL in WHERE clause when SELECT is used?

To address your specific issue of dealing with null values, you can simply use isnull

SELECT *
  FROM MyTable
 WHERE PartNum   = (SELECT isnull(Value, PartNum) FROM #Variables WHERE VarName = 'PartNum')
   AND PartColor = (SELECT isnull(Value, PartColor) FROM #Variables WHERE VarName = 'PartColor')

How to check a column values is null or not in select statement

Let me assume that blank is NULL. In that case, you simply want coalesce():

select coalesce(COM_TYPE, COM_TYPE_OTHERS, COM_IN_CHARGE_POSITION, COM_IN_CHARGE_POSITION_OTHERS) as effective_COM_TYPE

The coalesce() function takes the first non-NULL argument. If the values could be empty strings or strings with spaces in addition to NULL, then a case expression is recommended.

MYSQL - SELECT NULL otherwise latest DATE per group

count(*) counts all rows, count(date) counts non-null values. If they are the same, no null exists, i.e. return max(date), else return null.

SELECT MODEL, case when count(*) = count(DATE) then max(DATE) end
FROM MODEL_TABLE
group by MODEL

sql: select where problem with null value

Presumably, you want:

SELECT * from students WHERE name <> 'John' or name is null

Assuming that you are running MySQL, as your screen copy suggests, you could also express with the null-safe operator

SELECT * from students WHERE NOT name <=> 'John'

NULL usage in WHERE IN SQL statement

If you set ANSI_NULLS OFF first, you can use IN (Null) Fine.

Comparisons to NULL can't be performed (=, >, < etc) with ANSI_NULLS ON

SET ANSI_NULLS OFF
Select 
RPAD(x.QUOTE_ID,20,' ')
from csa_sli_all.T_CONV_XREF_CUST_QUOTE x ,
        csa_sli_all.T_CONV_quote q
where   q.select_indicator is null and 
    q.QUOTE_ID = X.QUOTE_ID and 
    q.HOLD_CODE IN ('CAQ' , NULL )

Should work fine

SQL select earlier date (including NULL)

You can have:

Select Case When Column1 is null then Column2 when Column2 is Null then Column1 When Column1 > Column2 Then Column2 When Column1 < Column2 Then Column1 End As EarlierDate From TableName

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