Is there any difference between MySQL
IF (myText IS NOT NULL) THEN
and
IF (myText != '') THEN
Yes there is a big difference between a NULL value and a blank/empty value.
Here's one resource that describes the differences.
When myText IS NULL:
myText IS NOT NULL evaluates to FALSE
myText != '' evaluates to NULL (which essentially behaves the same as FALSE would in this specific case you wrote)
However, you should not get into the habit of treating them the same, since most of the time they will behave differently: For example:
Assume you have a table tbl:
id text
1 NULL
2
3 abc
Note: 1 contains a NULL value, and 2 contains an empty string ('').
If you run the following query:
SELECT * FROM tbl WHERE text != ''
... it will return record 3.
If you run the following query:
SELECT * FROM tbl WHERE text IS NOT NULL
... it will return records 2 and 3.
Yes there is a difference.
In simple words, myText IS NOT NULL specifies that myText is having some value which could be '' too.
Where as myText != '' specifies that it returns TRUE, if myText does NOT contain an empty string.
There is a difference. If the default value of a column is "NULL", then if no data has been set for a field, it is truly null. However, if the value of a field has been updated as '', it is not NULL, rather it is empty.
See here for more information Link
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"
In this below example why other than "NAME1" all giving null as result in oracle 11g.
If I mention space explicitly it takes space otherwise null only not empty string.Please help me to clarify this.
In NAME2 I specify the empty space but still it gives null.
select
NVL(NAME,' ') AS NAME1,
NVL(NAME,'') AS NAME2,
NVL(NAME,NULL) AS NAME3,
NAME AS NAME4
from employee
OUTPUT :
(space),null,null,null
Because in Oracle a ZERO length varchar is treated as NULL.
In your example
NVL(NAME, ' ') AS NAME1 will evaluate to either NAME or ' ' - empty string.
NVL(NAME, '') as NAME2 will evaluate to either NAME or a zero length string
Because '' is equal with null in oracle. For example these two queries are same:
update test set name='' where id = 15
update test set name=null where id = 15
The reason is that in Oracle an empty string is equivalent to NULL.
You can see that this is true by executing the followig query
SELECT 'x' FROM DUAL WHERE '' IS NULL
This should return a single row, meaning that the condition '' IS NULL is true.
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';