SQL request execution take to much time - sql

Question

I have a SQL query which take sometimes 15 sec, sometimes 1 min.
Please, tell me how i can do my query lighter.
SELECT TOP 100
u.firstName,
u.id as userID,
ueh.targetID,
ueh.opened,
ueh.emailID,
u.phone,
u.gender
FROM dbo.Students u
INNER JOIN dbo.tblEmailHistory ueh
ON ueh.studentID = u.ID
WHERE (CONVERT(date,DATEADD(day,6,ueh.sDate))=CONVERT(date,getdate()))
AND IsNull(u.firstName, '') != ''
AND IsNull(u.email, '') != ''
AND IsNull(u.phone, '') != ''
AND ueh.status = 'sent'
AND ueh.reject_reason = 'null'
AND ueh.targetID = 28
AND ueh.opened = 0
AND u.deleted = 0
AND NOT EXISTS (SELECT ush.smsSendFullDate, ush.studentID FROM dbo.UsersSmsHistory ush WHERE u.id = ush.studentID AND DATEDIFF(DAY,ush.smsSendFullDate,GETDATE()) = 0)

Answer 1

This is your query greatly simplified:
SELECT TOP 100 u.firstName, u.id as userID,
ueh.targetID, ueh.opened, ueh.emailID,
u.phone, u.gender
FROM dbo.Students u INNER JOIN
dbo.tblEmailHistory ueh
ON ueh.studentID = u.ID
WHERE ueh.sDate >= cast(getdate() + 6 as date) AND
ueh.sDate < csat(getdate() + 7 as date) AND
u.firstName <> '' AND
u.email <> '' AND
u.phone <> '' AND
ueh.status = 'sent' AND
ueh.reject_reason = 'null' AND -- sure you mean a string here?
ueh.targetID = 28 AND
ueh.opened = 0 AND
u.deleted = 0 AND
NOT EXISTS (SELECT ush.smsSendFullDate, ush.studentID
FROM dbo.UsersSmsHistory ush
WHERE u.id = ush.studentID AND
convert(date, ush.smsSendFullDate) = convert(date, GETDATE())
);
Note: Comparisons to NULL are never true for almost all comparisons, so ISNULL()/COALESCE() is unnecessary.
Then start adding indexes. I would recommend:
tblEmailHistory(targetid, status, opened, deleted, rejectreason, sdate)
UsersSmsHistory(studentID, smsSendFullDate)
I am guessing most students have names and phone numbers, so indexes on those columns would not help.

Answer 2

Your query looks okay with no redundant parts. The reason it takes a lot of time is because you are joining tables three times and there may be a lot of data in them. So instead of improving your query, try to improve the performance of the table by adding index to them on columns like dbo.tblEmailHistory.studentID, dbo.Students.ID, etc