How to convert inline SQL queries to JOINS in SQL SERVER to reduce load time - sql

Question

I need help in optimizing this SQL query.
In the main SELECT statement there are three columns which is dependent on the outer query result. This is why my query is taking a long time to return data. I have tried making left joins but this is not working properly.
Can anyone help me to resolve this issue?
SELECT
DISTINCT ou.OrganizationUserID AS StudentID,
ou.FirstName,
ou.LastName,
(
SELECT
STRING_AGG(
(ug.UG_Name),
','
)
FROM
Groups ug
INNER JOIN ApplicantUserGroup augm ON augm.AUGM_UserGroupID = ug.UG_ID
WHERE
augm.AUGM_OrganizationUserID = ou.OrganizationUserID
AND ug.UG_IsDeleted = 0
AND augm.AUGM_IsDeleted = 0
) AS UserGroups,
order1.OrderNumber AS OrderId -- UAT-2455
,
(
SELECT
STRING_AGG(
(CActe.CustomAttribute),
','
)
FROM
CustomAttributeCte CActe
WHERE
CActe.HierarchyNodeID = dpm.DPM_ID
AND CActe.OrganizationUserID = ps.OrganizationUserID
) AS CustomAttributes -- UAT-2455
,
(
SELECT
STRING_AGG(
(CActe.CustomAttributeID),
','
)
FROM
CustomAttributeCte CActe
WHERE
CActe.HierarchyNodeID = dpm.DPM_ID
AND CActe.OrganizationUserID = ps.OrganizationUserID
) AS CustomAttributeID
FROM
ApplicantData acd WITH (NOLOCK)
INNER JOIN ClientPackage ps WITH (NOLOCK) ON acd.ClientSubscriptionID = ps.ClientSubscriptionID
INNER JOIN [ClientOrder] order1 WITH (NOLOCK) ON order1.OrderID = ps.OrderID
AND order1.IsDeleted = 0
INNER JOIN OUser ou WITH (NOLOCK) ON ou.OrganizationUserID = ps.OrganizationUserID

Answer 1

It looks like this query can be simplified, and the dependent subqueries in your SELECT clause removed, Consider your second and third dependent subqueries. You can refactor them into one nondependent subquery with a LEFT JOIN. Using nondependent subqueries is more efficient because the query planner can run them just once, rather than once for each row.
You want two STRING_AGG() results from the same table. This subquery gives those two outputs for every possible combination of HierarchyNodeID and OrganizationUserID values. STRING_AGG() is an aggregate function like SUM() and so works nicely with GROUP BY.
SELECT HierarchyNodeID, OrganizationUserID,
STRING_AGG((CActe.CustomAttribute), ',') CustomAttributes -- UAT-2455,
STRING_AGG((CActe.CustomAttributeID), ',') CustomAttributeIDs -- UAT-2455
FROM CustomAttributeCte CActe
GROUP BY HierarchyNodeID, OrganizationUserID
You can run this subquery itself to convince yourself it works.
Now, we can LEFT JOIN that into your query. Like this. (For readability I took out the NOLOCKs and used JOIN: it means the same thing as INNER JOIN.)
SELECT DISTINCT
ou.OrganizationUserID AS StudentID,
ou.FirstName,
ou.LastName,
'tempvalue' AS UserGroups, -- shortened for testing
order1.OrderNumber AS OrderId, -- UAT-2455
uat2455.CustomAttributes, -- UAT-2455
uat2455.CustomAttributeIDs -- UAT-2455
FROM ApplicantData acd
JOIN ClientPackage ps
ON acd.ClientSubscriptionID = ps.ClientSubscriptionID
JOIN ClientOrder order1
ON order1.OrderID = ps.OrderID
AND order1.IsDeleted = 0
JOIN OUser ou
ON ou.OrganizationUserID = ps.OrganizationUserID
LEFT JOIN (
SELECT HierarchyNodeID, OrganizationUserID,
STRING_AGG((CActe.CustomAttribute), ',') CustomAttributes -- UAT-2455,
STRING_AGG((CActe.CustomAttributeID), ',') CustomAttributeIDs -- UAT-2455
FROM CustomAttributeCte CActe
GROUP BY HierarchyNodeID, OrganizationUserID
) uat2455
ON uat2455.HierarchyNodeID = dpm.DPM_ID
AND uat2455.OrganizationUserId = ps.OrganizationUserID
See how we collapsed your second and third dependent subqueries to just one, then used it as a virtual table with LEFT JOIN? We transformed the WHERE clauses from the dependent subqueries into an ON clause.
You can test this: run it with TOP(50) and eyeball the results.
When you're happy, the next step is to transform your first dependent subquery the same way.
Pro tip Don't use WITH (NOLOCK), ever, unless a database administration expert tells you to after looking at your specific query. If your query's purpose is a historical report and you don't care whether the most recent transactions in your database are represented exactly right, you can precede your query with this statement. It also allows the query to run while avoiding locks.
SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED;
Pro tip Be obsessive about formatting your queries for readability. You, your colleagues, and yourself a year from now must be able to read and reason about queries like this.