[Solved] How to improve performance of this SQL Server query?
Looking to automatically optimize YOUR SQL query? Start for free.

EverSQL Database Performance Knowledge Base

How to improve performance of this SQL Server query?

Database type:

I was asked this question at web developer interview. after my answer interviewer said your in second table :(

I have two tables employee and bademployee:

Now, I want to select only good employees.

My answer was :

SELECT * 
FROM employee 
WHERE empid NOT IN (SELECT badempid from bademployee)

He said this query is not good for performance.

Can any one tell me how to write query for same result, by not using negative terms(not in, !=).

Can it be done using LEFT OUTER JOIN ?

How to optimize this SQL query?

The following recommendations will help you in your SQL tuning process.
You'll find 3 sections below:

  1. Description of the steps you can take to speed up the query.
  2. The optimal indexes for this query, which you can copy and create in your database.
  3. An automatically re-written query you can copy and execute in your database.
The optimization process and recommendations:
  1. Avoid Selecting Unnecessary Columns (query line: 2): Avoid selecting all columns with the '*' wildcard, unless you intend to use them all. Selecting redundant columns may result in unnecessary performance degradation.
  2. Create Optimal Indexes (modified query below): The recommended indexes are an integral part of this optimization effort and should be created before testing the execution duration of the optimized query.
  3. Replace In Subquery With Correlated Exists (modified query below): In many cases, an EXISTS subquery with a correlated condition will perform better than a non correlated IN subquery.
Optimal indexes for this query:
CREATE INDEX bademployee_idx_badempid ON bademployee (badempid);
The optimized query:
SELECT
        * 
    FROM
        employee 
    WHERE
        NOT EXISTS (
            SELECT
                1 
            FROM
                bademployee 
            WHERE
                (
                    employee.empid = bademployee.badempid
                )
        )

Related Articles



* original question posted on StackOverflow here.