i get confused in learning coalesce, I am new to sql.
example :
select case when value is null then 1
else value end as value
from table
and
select coalesce(value, 1)
from table
and in the tutorial I see in internet there are like
select coalesce (arg_1, arg_2, arg_3)
if I make
select coalesce(value, 1, 2)
how I can make to show the return value is 2?
Your query with the first and second will reproduce the same result, But you are wrong understanding the Coalesce concept.
Definition in Documentation Postgresql
The COALESCE function returns the first of its arguments that is not
null. Null is returned only if all arguments are null.
So it means it will return the first argument that is not null, it is not like case statement with condition like true or false
Let's try with example :
select coalesce(null, 1)
It will return 1 like the query you show, or
select coalesce(null, null, 1)
It will return 1 too even 1 in the arg_3 and how about there are 2 value not null?
select coalesce(null, 1, 2)
It will return 1. Why? Like in the documentation said "returns the first of its arguments that is not null" so when there is 2 value not null the first argument have not null value will get return
You can check this demo and try :
Demo<>Fiddle
Hope it helps
Related
I have the following condition:
if column is NULL, or have value like Unassessed or ABC then the result should be I.
I need help how to put three in one condition like below. Just confused with NULL value. And it is string in column.
JobAssessStage ='Unassessed','','ABC'
JobAssessStage in ('Unassessed','','ABC')
Use below condition with is null
JobAssessStage in ('Unassessed','ABC') or JobAssessStage is null
use case when for multiple condition
select case when JobAssessStage in ('Unassessed','ABC') or
JobAssessStage is null then 'I' else 'You' End
from yourtable
In some cases you can use also use COALESCE function (it returns the first non-null expression in a list) on JobAssessStage field:
COALESCE(JobAssessStage, 'X') in ('Unassessed','ABC','X')
If JobAssessStage is NULL with COALESCE the value you're testing becomes X.
I told you "in some cases" because you have to be sure that the value you have chosen (X) is not a value that your field can take.
You can formulate your like below:
SELECT *
FROM table
WHERE 1 = 1
AND (JobAssessStage IN ('Unassessed', 'ABC')
OR JobAssessStage IS NULL);
I am using Postgres and have the following SQL statement:
SELECT *
FROM "osmlocal-dsd-de".t_osm_vehicle_image t
WHERE t.vehicle_config_id = 3
and image_type_id = 2
Which returns one row:
id vehicle_config_id cosy_url image_type_id
113 3 SomeValue 2
When I run the following:
SELECT * from "osmlocal-dsd-de".t_osm_vehicle_image t
WHERE t.vehicle_config_id = 3
and image_type_id = 2
and coalesce(t.cosy_url, '') = ''
Zero rows are returned.
I think my understanding of coalesce is wrong, because I would have expected one row still to be returned, because the cosy_url is not null.
Any advise on what I am doing wrong would be appreciated.
Your understanding of coalesce is wrong
It returns the first argument that is not null. If all arguments are null, the COALESCE function will return null
In your case t.cosy_url is not null it is equally SomeValue and your condition doesn't work because SomeValue is not equal ''
You seem to be misunderstanding coalesce(). It returns the first value that is not null.
In your case, you have:
coalesce(t.cosy_url, '')
Because t.cosy_url has a value ('SomeValue'), this evaluates to that value. The value is not '' so the expression returns false and the entire where clause returns false.
If you want non-NULL values, then use:
t.cosy_url is not null
I have a table with about a hundred rows. It has a column is_gallery that contains either 1, 0, or NULL. If I do...
SELECT * WHERE is_gallery != 1
or
SELECT * WHERE NOT (is_gallery = 1)
it excludes the rows where is_gallery is null. I can manage to get a proper response if I do
SELECT * WHERE (is_gallery = 0 OR is_gallery is null)
But shouldn't the "!=" or NOT work? Isn't there a way to just return the rows where is_gallery doesn't equal 1 without testing for every other possibility?
You can use the IS and IS NOT operators instead of = and !=. These treat NULL like a normal value.
SELECT * FROM yourTable WHERE is_gallery IS NOT 1
The best thing to use is coalesce as in:
SELECT *
WHERE coalesce(is_gallery,0) != 1;
what coalesce does, is replaces any null value in that column with the second parameter. In the example above, any nulls in the "is_gallery" column will be replaced with 0 before it is compared with 1. So will of course return true.
On NULL realize that a NULL value isn't equal to ANYTHING - not even NULL itself. It cannot be compared - so when "comparing", it always will return FALSE. On NULL, it has a special operator which is "IS NULL" or "IS NOT NULL"
How can I return 1 if the SQL below returns NULL?
Something like (pseudo code):
if sql return NULL value
then set value to one
otherwise returning sql result value.
Is there any SQL for defining default value to 1 if SQL result is NULL?
SELECT Max(iCategoryOrder)+1
FROM [IVRFlowManager].[dbo].[tblCategory]
WHERE iCategoryLevel = 1
Option 1
Use ISNULL()
Description
Replaces NULL with the specified replacement value.
SELECT MAX(ISNULL(iCategoryOrder, 0))+1
FROM [IVRFlowManager].[dbo].[tblCategory]
WHERE iCategoryLevel = 1
Option 2
Use COALESCE()
SELECT MAX(COALESCE(iCategoryOrder, 0))+1
FROM [IVRFlowManager].[dbo].[tblCategory]
WHERE iCategoryLevel = 1
Description
Returns the first nonnull expression among its arguments.
Use ISNULL operator like:
ISNULL(your_field, 1)
Try following:
Select ISNULL(Max(iCategoryOrder), 0) + 1
from [IVRFlowManager].[dbo].[tblCategory]
where iCategoryLevel = 1
I have a row full of 1, 2, 3 and null. Why don't there two return the same result:
select * from foo where foobar not in (1, 2, 3);
select * from foo where foobar is not null;
The first one returns empty set, while second one works as advertised. Now I'm a bit confused :-D Is there some kind of "NULL in SQL for newbies" document anywhere? :-D
(I'm using Oracle, if that matters)
It's because your first statement is being evaluated like this:
select * from foo
where foobar <> 1 and foobar <> 2 and foobar <> 3
"null <> 1" evaluates to null, not true/false, so nothing is returned.
Any boolean operation involving NULL always fails. If foobar is NULL then:
- The test = 1 fails.
- But <> 1 also fails.
This means that NULL not in (1, 2, 3) always fails.
It's a peculiarity of NULL Logic that takes getting used to at first. NULL can be taking to mean Unknown. Which means it might be 1, an might might not. It's indeterminate.
So when you ask is NULL in the list (1, 2, 3) the answer is I can't tell, it's indeterminate. So rather than being TRUE or FALSE the answer itself is NULL.
And because NULL isn't TRUE, the row has failed the test.
not in is not opposite of in
for detail check http://jonathanlewis.wordpress.com/2007/02/25/not-in/