SQL Server: breaking a multiple join query into smaller ones and creating a function - sql-server

Question

I have a complex query with multiple joins in it, which runs multiple times in my application. I want to write this query as a function by breaking this query into smaller pieces inside the function. As a newbie, I have limited knowledge on SQL Server.
The following is the query:
SELECT
ts.lable as label,
ts.percentage as rate
FROM
TaxSet ts
JOIN
UserInfo u ON u.userID = ?
AND u.countryID = ts.countryId
AND (ts.stateId IS NULL OR ts.stateId = 0 OR LEN(ts.stateId) < 1)
JOIN
Users us ON u.userID = us.id
JOIN
Users p ON us.parentID = p.id
AND ts.ispID = p.id
JOIN
ProductType pt ON ts.productTypeID = pt.id
WHERE
startDate <= getutcdate()
AND getutcdate() <= endDate
AND pt.identifier = ?
AND ts.id NOT IN (SELECT eu.ispTaxSettingId
FROM ExemptUsers eu
WHERE eu.ExemptUserId = ?)
Now, how can I write a function by breaking this query into smaller ones.
Thanks in advance.

Answer 1

May I ask why you want to split it into functions? I reformatted your code and have put it into a stored procedure for now. My thinking is that you want to pass through a Identifier and UserID which are the parameters of your query.
I have modified the query and removed the Not In statement. This has been replaced by a LEFT JOIN to ExemptUsers on u.id = eu.ExemptUserID and then an addition to the WHERE clause to ensure eu.ExemptUserID is NULL. This is basically a clearer way of saying "If the userID exists in table ExemptUsers do not bring back results for that user".
In addition I have removed the join to Users p as I can't see that this was being used in any way, unless you want to ensure that the user has a parent?
CREATE PROCEDURE wsp_StoredProcName
(#UserID int,
#Identifier int)
AS
BEGIN
SELECT
ts.lable as label,
ts.percentage as rate
FROM
TaxSet ts
INNER JOIN UserInfo u ON u.userID = ts.UserID
AND u.countryID = ts.countryId
INNER JOIN Users us on u.userID = us.id
INNER JOIN ProductType pt on ts.productTypeID = pt.id
LEFT JOIN ExemptUsers eu on u.id = eu.ExemptUserID
WHERE
(
ts.UserID = #UserID
and pt.identifier = #Identifier
and startDate <= getutcdate()
and getutcdate() <= endDate
AND eu.ExemptUserID IS NULL
and
(
ts.stateId is null or ts.stateId = 0 or len(ts.stateId) < 1
)
)
END

Answer 2

After all you select from TaxSet where certain conditions must be met: The date range, the state, a relation to a particular user and its parent, a relation to a particular product type, and the non-existence for a particular exempt user. So use EXISTS and NOT EXISTS throughout your query in order to make it plain to the reader and dbms what you want to achieve. The more straight-forward a query, the easier it often is for the optimizer to deal with it.
select
ts.lable as label,
ts.percentage as rate
from taxset ts
where getutcdate() between ts.startdate and ts.enddate
and (stateid is null or stateid between 0 and 9)
and exists
(
select *
from users u
join userinfo ui on ui.userid = u.id
where u.id = ?
and ui.countryid = ts.countryid
and u.parentid = ts.ispid
)
and exists
(
select *
from producttype pt
where pt.identifier = ?
and pt.id = ts.producttypeid
)
and not exists
(
select *
from exemptusers eu
where eu.exemptuserid = ?
and eu.isptaxsettingid = ts.id
);
As others have mentioned: When splitting a query into smaller parts and execute these separately, it usually becomes slower not faster. This is due to the fact that a dbms is made to do exactly this in the most efficient way internally. Of course sometimes a dbms' optimizer fails to find a good execution plan, though. You may want to look at the execution plan and check whether you find that plan appropriate.