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
Related
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
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"
I am having trouble with some sql. When I run the following query:
Select * from teleapp;
I get TONS of results. Resulst which include a column (called cashwithappyn) that has TONS of empty or null data cells. (They look empty and don't say null)
The column info is:
ColumnName ID Null? Data Type Histogram Num Distinct Num Nulls Density
CASHWITHAPPYN 54 Y VARCHAR2(1 Byte) Frequency 2 56895 0
When I try to run the following query:
Select * from teleapp where cashwithappyn = null;
or
Select * from teleapp where cashwithappyn = '';
or
Select * from teleapp where cashwithappyn not like '';
or
Select * from teleapp where cashwithappyn not in ('Y','N');
or ANY type of combination, I cannot seem to get all of the rows with nothing in cashwithappyn.
Any ideas? Please help, this is the last part of a project that I was assigned to do and I just need to figure this out.
Thanks.
Maybe the column contains blanks. In that case you can do
WHERE TRIM(CASHWITHAPYYN) IS NULL
TRIM removes all blanks before and after and if nothing is left anymore the value becomes NULL
e.g.
TRIM(' ') IS NULL -- one blank removed = true
TRIM(NULL) IS NULL -- true
Also NULL cannot be compared with = NULL but must be phrased IS NULL. NULL is not a value as such and that is why the comparison never works.
You need to use IS NULL
Select * from teleapp where cashwithappyn is null;
Logical test expressions (=, Not In, Like etc) with null result in a false so all of the following result in false
1 = NULL
1 <> NULL
NULL = NULL
NULL <> NULL
NULL NOT IN ('a','b')
NULL Not Like NULL
Additionally in oracle the zero length string is null so NOT LIKE '' will never return any rows
You'll need to use either is null, is not null or Coalesce
A co-worker and I did a bunch of research, here's what we came up with that will work. Any ideas why this works but not the others?
Select * from teleapp where nvl(cashwithappyn,'U') = 'U';
I was curious if it's legal in t-SQL to compare a NULL to a value?
For instance, if I have:
WITH ctx AS(SELECT 123 AS n0, NULL AS n1)
SELECT n0 FROM ctx
WHERE ctx.n1 < 130
the WHERE clause in that case is always evaluated as FALSE. Is it something I can rely on?
You can't compare NULL with any other value, it will result in 'UNKNOWN'.
From msdn source
A value of NULL indicates that the value is unknown. A value of NULL
is different from an empty or zero value. No two null values are
equal. Comparisons between two null values, or between a NULL and any
other value, return unknown because the value of each NULL is unknown.
All boolean operations in T-Sql with null value returns 'UNKNOWN', which is recognized as false in clauses. You can use ISNULL function when you want set some default value.
for example in your case:
WITH ctx AS(SELECT 123 AS n0, NULL AS n1)
SELECT n0 FROM ctx
WHERE isnull(ctx.n1,0) < 130
It depends on the value of ANSI_NULLS.
http://msdn.microsoft.com/en-us/library/ms191270%28v=sql.90%29.aspx
When SET ANSI_NULLS is ON, a comparison in which one or more of the
expressions is NULL does not yield either TRUE or FALSE; it yields
UNKNOWN.
Transact-SQL supports an extension that allows for the comparison
operators to return TRUE or FALSE when comparing against null values.
This option is activated by setting ANSI_NULLS OFF. When ANSI_NULLS is
OFF, comparisons such as ColumnA = NULL return TRUE when ColumnA
contains a null value and FALSE when ColumnA contains some value
besides NULL.
The WHERE clause in the following = is also FALSE. You need to be very careful with NULLs
WITH ctx AS
(
SELECT 123 AS n0, NULL AS n1
)
SELECT *
FROM ctx
WHERE ctx.n1 = NULL
I've always used the EXISTS keyword along with EXCEPT like so
SELECT 1
WHERE EXISTS ((SELECT 1) EXCEPT (SELECT 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