Tuning a query converting subquery

I was trying to change an update statement where there is a subquery in the WHERE clause to a join in order to improve performance. DDL to sample table/data

CREATE TABLE [dbo].[Table1](
[pre_plan_id] [smallint] NULL,
[pre_type_id] [smallint] NULL,
[associate_id] [smallint] NOT NULL,
[deleted] [bit] NOT NULL
)  

INSERT INTO Table1 
VALUES 
(NULL,  NULL,   -32768, 0),
(1,  NULL,   2,  1),
(1,  NULL,   3,  0), 
(NULL, NULL, 3109, 0), 
(1, NULL, 3109, 1) 

CREATE TABLE [dbo].[Table2](
[type_id] [smallint] NOT NULL,
[plan_id] [smallint] NOT NULL,
[associate_id] [smallint] NOT NULL,
[time_in] [smalldatetime] NOT NULL
) 
INSERT INTO Table2 
VALUES
(390,   31, 3109,   '2009-09-02'),
(304,   32, 3109,   '2010-02-05'),
(388,   31, 3109,   '2010-09-24') 

The query that uses a subquery:

SELECT pre_plan_id, pre_type_id FROM Table1  WHERE pre_plan_id =1 
AND associate_id  NOT IN 
(SELECT TOP 2 associate_id 
FROM Table2 WHERE time_in= '2010-09-24 00:00:00' group by associate_id order by count(*) desc) 

My attempt of converting it to JOIN

SELECT pre_plan_id 
FROM (SELECT pre_plan_id, pre_type_id, rn
FROM Table1 a  
LEFT JOIN 
( select associate_id, Row_number() over (partition by associate_id order by count(*) desc ) rn 
FROM Table2 
WHERE time_in= '2010-09-24 00:00:00' Group by associate_id) b 
ON a.associate_id = b.associate_id where b.rn <> 1) a
where pre_plan_id = 1  

However, this displays nothing while I expect two rows; and it is happening because of b.rn <> 1 I expected it would display the NULL values when it is b.rn <> 1

Any explanation for this? Any guide on better approach of tuning the query is much appreciated.

Thanks.

Answer

You can just move the subquery to the FROM clause and use LEFT JOIN:

SELECT t1.pre_plan_id, t1.pre_type_id
FROM Table1 t1 LEFT JOIN
     (SELECT TOP 2 t2.associate_id 
      FROM Table2
      WHERE t2.time_in= '2010-09-24 00:00:00' 
      GROUP BY associate_id 
      ORDER BY COUNT(*) DESC
     ) t2
     ON t2.associate_id = t1.associate_id
WHERE t1.pre_plan_id = 1 AND t2.associate_id IS NULL;