SQL Server performance tip - Do not use NOT IN clause in Subquery use LEFT OUTER JOIN instead

Question

CREATE TABLE Employees(EmpID INT,Name VARCHAR(50))
INSERT INTO Employees VALUES(1,'Steve')
INSERT INTO Employees VALUES(2,'Brown')
INSERT INTO Employees VALUES(3,'Smith')
INSERT INTO Employees VALUES(4,'Chris')
INSERT INTO Employees VALUES(5,'Andrew')

CREATE TABLE ResginedEmployees(EmpID INT,Date DATETIME)
insert into ResginedEmployees VALUES(3,'2008-01-01')
insert into ResginedEmployees VALUES(4,'2009-01-01')

Normal query to get all employees who are not resigned is,

SELECT * FROM Employees WHERE EmpID NOT IN (SELECT EmpID FROM ResginedEmployees)

This query execution time would degrade the performance. The best way to write the query for the same result is use LEFT OUTER JOIN and use NULL value to any column of second table in where condition as shown below.
SELECT * FROM Employees E LEFT OUTER JOIN ResginedEmployees R on E.EmpID=R.EmpID
WHERE R.EmpID is NULL

Answer 1

Thanks for the tip

Answer 2

thanks for that information

Answer 3

Thanks for the heads up

Answer 4

🤗

Answer 5

nice tip bro