This question already has answers here:
SQL is null and = null [duplicate]
(4 answers)
Closed 8 years ago.
I am trying to count the number of missing postal codes using CASE to flag null values as 1, otherwise 0.
CASE WHEN ([postcode]=NULL)
THEN 1
ELSE 0 END AS pc_missing
Even though I know there are lots of missing data, none of them receive the value of 1.
What have I done wrong?
null is not a value - it's the lack of a value. null returns "unknown" (which is not true) when evaluated against any value operator - =, !=, >, <, etc. Instead, you should check it with the is [not] null operator:
CASE WHEN ([postcode] IS NULL)
THEN 1
ELSE 0 END AS pc_missing
Related
This question already has answers here:
SQL is null and = null [duplicate]
(4 answers)
Closed 11 months ago.
I use SQL inside a 3rd party system (So don't know the type it is)
I am trying to make a CASE work on a column using data from 2 more.
I want to display a column call Channel that is calculated using the following logic:
If column O.Test is blank and column o.subsource is not blank, display 'RESEND', otherwise display the value of column o.Source.
This is part of the SQL showing the CASE I wrote to do this:
select
-- other columns
(CASE
WHEN o.Test = NULL AND o.Subsource IS NOT NULL THEN 'RESEND'
ElSE o.Source
END) o.Source AS 'Channel',
-- other columns
The SQL runs with no errors but the output always shows what is in o.Source.
X = NULL is not equal IS NULL X
Check this question,
SQL is null and = null
change this statement "o.Test = NULL" instead of "o.Test IS NULL"
This question already has answers here:
Issues with SQL comparison and null values
(8 answers)
Closed 2 years ago.
I just realized very strange behavior of my MsSql database when filtering empty string != '' or <> '' regarding NULL values.
I have following data in the table
ID
Value
(table X)
1
(empty string)
2
NULL
(no value)
3
text
(some real text)
This query select * from X where Value = '' results in:
ID: [1]
Both queries select * from X where Value != '' and select * from X where Value <> '' result in:
ID: [3]
What I do not understand is, why 2nd query is not returning ID=2?
I know the syntax for checking explicitly on null values where Value IS NULL so I would expect, that 2nd query would behave differently. When checking for non-empty values, I used to write where Value <> '' AND Value IS NOT NULL. From now on I am totally confused...
NULL does not have a string value - or any value at all, whereas ' ' is an empty string. You're looking for not empty strings in your 2nd query. NULL is not an empty string or a string at all. NULL is a "value" of no other type than NULL itself
It is not possible to test for NULL values with comparison operators,
such as =, <, or <>.
https://www.w3schools.com/sql/sql_null_values.asp
This question already has answers here:
What is NULL in SQL?
(5 answers)
Closed 4 years ago.
What is the difference between initializing a number type variable by null and by Zero in PLSQL?
example-
l_person_id NUMBER :=NULL;
and
l_person_id NUMBER :=0;
Does this effect anywhere in a code?
There is a difference.
If you have comparison operations in your code then you can get unexpected behaviour:
5 > l_person_id_null -- false
5 > l_person_id_0 -- true
Also, with NULL you'll have to use IS NULL and IS NOT NULL operators instead of =.
This question already has answers here:
why is null not equal to null false
(7 answers)
Closed 6 years ago.
I guess I don't fully understand the meaning of NULL in SQL. I ran this SQL and I expected to see 1 as the output but I didn't see that:
select 1 where NULL <> -1;
Isn't NULL and -1 different? Can anyone explain why this clause of "NULL <> -1" is FALSE?
NULL Is not a value and therefore cannot be compared with any other value and get any other result than null. Docs. And how to compare nulls.
NULL means unknown. So with where NULL <> -1 you want to know whether the unknown value equals -1. The DBMS does not know (of course), so the result of the expression is neither TRUE nor FALSE; it is NULL.
Only rows for which the WHERE clause results in TRUE are selected. As your expression doesn't result in TRUE but in NULL, there is no row selected.
This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
Oracle <> , != , ^= operators
in sql, what is the difference between <> and !=, we can use both for "NOt Equal to".
is there any difference in between them?
ex.
select * from student where no != 2;
&
select * from student where no <> 2;
is any advantage of using one insted of another?
What are the main factors because of which != is not made as ISO standard
For SQL Server:
They are the same. Both are two Not Equal To operators. But != is not ISO standard, as quoted from Comparison Operators:
<> (Not Equal To) Not equal to
!= (Not Equal To) Not equal to (not ISO standard)