Replace ISNULL function in WHERE clause - sql

Question

I was asked to rewrite the below queries by eliminating the ISNULL Function in where clause due to performance issues. How can I rewrite the below queries by replacing the ISNULL Function in WHERE clause and in JOIN condition. Below are the sample queries.
Select *
from child c
inner join Parent P on c.id=p.id
where ISNULL(p.ParentId, 0) <> 0
Select *
from child c
inner join Parent P on isnull(c.Parentid,0)=p.id
select *
from parent
where isnull(Status, '') != 'Active'
Select *
from child c
inner join Parent P on c.id=p.id
where CAST(P.PostedDate AS DATE)
BETWEEN CAST(isnull(#FromDate,P.PostedDate) AS DATE)
AND CAST(#ToDate AS DATE)
SELECT *
FROM TEMP
WHERE EDATE=ISNULL(#EDATE,EDATE)

Answer 1

In most cases you can simply remove the isnull function, and it need not be replaced by anything. For example:
select *
from parent
where isnull(Status, '') != 'Active'
Can be rewritten as
select *
from parent
where Status != 'Active'
Since if Status is null, the comparison still returns FALSE. The rule is:
NULL = value: NULL (interpreted as FALSE, since it is not TRUE).
If NULL has to be included in your results, you can add Attribute/#Variable IS NULL.

Answer 2

Be carefull with this
Look at this example :
declare #table table(ID int, status varchar(50))
insert into #Table (ID, status)
values (1, 'active'), (2, ''), (3, null), (4, 'active')
select t.*
from #Table t
where t.status <> 'active'
This will return just one record (ID: 2)
while this
select t.*
from #Table t
where isnull(t.status, '') <> 'active'
will return 2 records (ID: 2 and 3)
So it depends on what you want in your result set, do you want records where the field Status is null also returned or not ?
In case you dont want them then the answer is simple
select t.*
from #Table t
where t.status <> 'active'
Otherwise you need something like this
Since the original query had IsNull in it, I suspect this is what you want.
select t.*
from #Table t
where (t.status is null or t.status <> 'active')
This does not use the IsNull() but I am not sure if much performance will be gained here.

Answer 3

Select * from child c inner join Parent P on c.id=p.id where ISNULL(p.ParentId, 0) <> 0
Here, neither you need ISNULL nor any alternative of this because in this case
NULL <> 0 will be FALSE similarly 0 <> 0 also FALSE
NULL = NULL will be FALSE if the setting SET ANSI_NULLS ON else TRUE
And here, the alternative of this is,
If you want to return NULL as TRUE then you can use
WHERE (p.ParentId IS NULL OR p.ParentId <> 0)
Otherwise,
(p.ParentId <> 0 OR 0 <> 0) --But it doesn't have any meaning and it's exactly same to simply use p.ParentId <> 0
You can handle ISNULL accordingly in your all the questions.

Answer 4

Please try like this.
Select * from child c inner join Parent P
on c.id=p.id OR where p.ParentId IS NULL
Select * from child c inner join Parent P on c.Parentid=p.id OR c.ParentId IS NULL
select * from parent
where [Status] != 'Active' AND [Status] IS NOT NULL
Select * from child c inner join Parent P on c.id=p.id
where
( (P.PostedDate) BETWEEN CAST(#FromDate) AS DATE) AND #FromDate IS NOT NULL AND CAST(#ToDate AS DATE) )
OR
( (P.PostedDate) BETWEEN P.PostedDate AND CAST(#ToDate AS DATE) )
SELECT * FROM TEMP
WHERE EDATE = CASE WHEN #EDATE IS NULL THEN EDATE ELSE #EDATE END

Answer 5

Substitute as below to make SARGABLE:
where ISNULL(p.ParentId, 0) <> 0 --- where p.ParentId = 0 or p.ParentID is null
Select * from child c inner join Parent P on isnull(c.Parentid,0)=p.id --- Select * from child c inner join Parent P on c.Parentid = p.id OR (c.Parentid is null and p.id = 0)
where isnull(Status, '') != 'Active' --- where Status <> 'Active'
where CAST(P.PostedDate AS DATE) BETWEEN CAST(isnull(#FromDate,P.PostedDate) AS DATE) AND CAST(#ToDate AS DATE) --- where P.PostedDate BETWEEN isnull(#FromDate,P.PostedDate) AND #ToDate
WHERE EDATE=ISNULL(#EDATE,EDATE) --- WHERE #EDATE IS NULL OR EDATE = #EDATE

Answer 6

When values can be null you have to use the Three-value logic.
Read this article.
So if you want to make your arguments SARGABLE avoiding to use ISNULL or COALESCE functions, you have to combine OR/AND/NOT/CASE operators, depending on logic of your query
In your examples:
Select *
from child c
inner join Parent P on c.id=p.id
-- where ISNULL(p.ParentId, 0) <> 0
where p.ParentId <> 0 -- when p.ParentId is null condition is evaluated as unknown and record are excluded.
Select *
from child c
-- inner join Parent P on isnull(c.Parentid,0)=p.id
inner join Parent P on (c.Parentid=p.id) OR (c.Parentid IS NULL AND p.id = 0)
select *
from parent
--where isnull(Status, '') != 'Active'
where Status IS NULL OR Status <> 'Active'
-- IN THIS CASE YOU HAVE NO BENEFIT BECAUSE CONVERSIONS MAKES YOUR ARGUMENTS NOT SARGABLE!!!
Select *
from child c
inner join Parent P on c.id=p.id
where CAST(P.PostedDate AS DATE)
BETWEEN CAST(CASE WHEN #FromDate IS NULL THEN P.PostedDate ELSE #FromDate END) AS DATE)
AND CAST(#ToDate AS DATE)
SELECT *
FROM TEMP
--WHERE EDATE=ISNULL(#EDATE,EDATE)
WHERE EDATE=#EDATE