Difference between IN and JOIN - sql

Question

I was wondering what was the difference between the two queries mentioned below, because the first one takes more than 10 seconds to execute on the server and the second one executes in less than one second...
UPDATE - I
Here are the actual queries and their Execution Plans as copied and pasted from SQL Server (as is), sorry for any inconvenience caused by my previous queries... :(
SELECT REPLACE(CONVERT(VARCHAR(11), m.PlanDate, 106), ' ', '-') AS ManagmentPlanDate
FROM ManagmentPlan m
INNER JOIN Product p ON p.Product_ID = m.ProductID
INNER JOIN Category c ON c.C_ID = p.C_ID
LEFT OUTER JOIN Employee e ON e.emp_no = m.PrescribedBy
LEFT OUTER JOIN dbo.Issue_Stock i ON i.serial_no = m.IssueStockID
INNER JOIN dbo.Units u ON u.U_ID = p.U_ID
WHERE ( ( #PatientID IS NULL )
AND ( #VisitID IS NULL )
AND ( m.WardRegNo = #WardRegNo )
)
OR --Get only cuurent admission TP
( ( #PatientID IS NULL )
AND ( #WardRegNo IS NULL )
AND ( VisitID = #VisitID
AND m.WardRegNo IS NULL
)
)
OR -- Get Only Current OPD visit TP
( ( #WardRegNo IS NULL )
AND ( #VisitID IS NULL )
AND ( visitid IN ( SELECT id
FROM PatientVisit
WHERE PatientID = #PatientID ) )
)
OR --Get All Visits TP
( ( #PatientID IS NULL )
AND ( #VisitID IS NOT NULL )
AND ( #WardRegNo IS NOT NULL )
AND ( ( VisitID = #VisitID )
OR ( m.WardRegNo = #WardRegNo )
)
) -- Get Current OPD visit and cuurent admission TP (Both)
AND m.Deleted != 1
AND m.PatientDeptID = #PatientDeptID
GROUP BY REPLACE(CONVERT(VARCHAR(11), m.PlanDate, 106), ' ', '-')
ORDER BY CAST(REPLACE(CONVERT(VARCHAR(11), m.PlanDate, 106), ' ', '-') AS DATETIME) DESC
and
SELECT REPLACE(CONVERT(VARCHAR(11), m.PlanDate, 106), ' ', '-') AS ManagmentPlanDate
FROM ManagmentPlan m
WHERE m.ProductID IN ( SELECT Product_ID
FROM Product
WHERE C_ID IN ( SELECT C_ID
FROM Category )
AND U_ID IN ( SELECT U_ID
FROM Units ) )
AND m.PrescribedBy IN ( SELECT Emp_no
FROM Employee )
AND m.IssueStockID IN ( SELECT Serial_No
FROM Issue_Stock )
AND ( ( #PatientID IS NULL )
AND ( #VisitID IS NULL )
AND ( m.WardRegNo = #WardRegNo )
)
OR --Get only cuurent admission TP
( ( #PatientID IS NULL )
AND ( #WardRegNo IS NULL )
AND ( VisitID = #VisitID
AND m.WardRegNo IS NULL
)
)
OR -- Get Only Current OPD visit TP
( ( #WardRegNo IS NULL )
AND ( #VisitID IS NULL )
AND ( visitid IN ( SELECT id
FROM PatientVisit
WHERE PatientID = #PatientID ) )
)
OR --Get All Visits TP
( ( #PatientID IS NULL )
AND ( #VisitID IS NOT NULL )
AND ( #WardRegNo IS NOT NULL )
AND ( ( VisitID = #VisitID )
OR ( m.WardRegNo = #WardRegNo )
)
) -- Get Current OPD visit and cuurent admission TP (Both)
AND m.Deleted != 1
AND m.PatientDeptID = #PatientDeptID
GROUP BY REPLACE(CONVERT(VARCHAR(11), m.PlanDate, 106), ' ', '-')
ORDER BY CAST(REPLACE(CONVERT(VARCHAR(11), m.PlanDate, 106), ' ', '-') AS DATETIME) DESC
Although, it solved my problem of speed or optimization of the query, but just was curious as to what exactly is the difference between those two queries, as I thought the first one translates to the second one...
UPDATE - I
As you can see, both queries differ in only the JOINS converted to IN statements...

Answer 1

For one, your first statement retrieves all matching records from both the Products and Category tables whereas your second statement only retrieves all matching rows from Products.
What is the performance difference if you change your first statement to
SELECT p.*
FROM Products p
INNER JOIN Category c ON p.CatNo = c.CatNo
Edit
(as mentioned by Martin) note that the number of rows is only identical for both statements if CatNo is unique in the Category table. The INNER JOIN will return as many records as there are in the Category table whereas the IN statement will return as many records as there are unique CatNo in the Category table.

Answer 2

An in clause filters the rows that come back from product. An inner join adds columns from category to the select statement output.