Except VS Not In SQL Server

Article

Introduction

EXCEPT operator was introduced in SQL Server in 2005. The EXCEPT operator returns all of the distinct rows from the query to the left of the EXCEPT operator when there are no matching rows in the right query. EXCEPT is similar as NOT IN with DISTINCT queries. The EXCEPT operator is equivalent of the Left Anti Semi Join.

NOT IN will return all rows from left hand side table which are not present in right hand side table but it will not remove duplicate rows from the result.

Let us take an example:

CREATE TABLE# NewData(ID INT, Name[nvarchar](30));
CREATE TABLE# ExistingData(ID INT, Name[nvarchar](30));
INSERT INTO# NewData
(ID, Name)
VALUES
(8, 'Pankaj'), (9, 'Rahul'), (10, 'Sanjeev'), (1, 'Sandeep'), (3, 'Priya'), (8, 'Deepak');
INSERT INTO# ExistingData
(ID, Name)
VALUES
(1, 'Sandeep'), (2, 'Neeraj'), (3, 'Priya'), (4, 'Omi'), (5, 'Divyanshu');
--EXCEPT filters
for DISTINCT values
SELECT nc.ID FROM# NewData AS nc
EXCEPT
SELECT ec.ID FROM# ExistingData AS ec
--NOT IN returns all values without filtering
SELECT nc.ID FROM# NewData AS nc
WHERE ID NOT IN(SELECT ec.ID FROM# ExistingData AS ec)
DROP TABLE# NewData;
DROP TABLE# ExistingData;

Output:

In above result first table is result of EXIST operator and second table is result of NOT IN operator. It is clear that EXIST operator always return the distinct result but NOT IN doesn’t remove duplicate value.

Let’s check the execution plan of both query.

Execution Plan

Execution plan of EXIST and NOTIN is almost similar, difference is that EXIST perform distinct sort for left side table.

If we performed the NOTIN operation with DISTINCT command then both query produce same result.

Example 1

SELECT nc.ID FROM# NewData AS nc
EXCEPT
SELECT ec.ID FROM# ExistingData AS ec
--NOT IN returns all values without filtering
SELECT DISTINCT nc.ID FROM# NewData AS nc
WHERE ID NOT IN(SELECT ec.ID FROM# ExistingData AS ec)

Output

Example 2

Now try to execute the following query and examine the output.

--EXCEPT filters for DISTINCT values
SELECT * FROM# NewData AS nc
EXCEPT
SELECT ec.ID FROM# ExistingData AS ec
--NOT IN returns all values without filtering
SELECT * FROM# NewData AS nc
WHERE ID NOT IN(SELECT ec.ID FROM# ExistingData AS ec)

Output

When we execute above query, SQL Server throw an error that “All queries combined using a UNION, INTERSECT or EXCEPT operator must have an equal number of expressions in their target lists”. This error indicate that number of column for both table (outer table and inner table) must be equal.

Example 3

Now first insert some NULL values for both columns in “#ExistingData” table and after that execute both query and examine the result.

CREATE TABLE# NewData(ID INT, Name[nvarchar](30));
CREATE TABLE# ExistingData(ID INT, Name[nvarchar](30));
INSERT INTO# NewData
(ID, Name)
VALUES
(8, 'Pankaj'), (9, 'Rahul'), (10, 'Sanjeev'), (1, 'Sandeep'), (3, 'Priya'), (8, 'Deepak');
INSERT INTO# ExistingData
(ID, Name)
VALUES
(1, 'Sandeep'), (2, 'Neeraj'), (3, 'Priya'), (NULL, NULL), (4, 'Omi'), (5, 'Divyanshu');
--EXCEPT filters
for DISTINCT values
SELECT nc.ID FROM# NewData AS nc
EXCEPT
SELECT ec.ID FROM# ExistingData AS ec
--NOT IN returns all values without filtering
SELECT nc.ID FROM# NewData AS nc
WHERE ID NOT IN(SELECT ec.ID FROM# ExistingData AS ec)
DROP TABLE# NewData;
DROP TABLE# ExistingData;

Output:

OOPS! What is going wrong ? Why doesn’t exist any result for NOT IN command ?

We know that NOT IN command just work like an “AND” operator. It means,

WHERE ID NOT IN (SELECT ec.ID FROM #ExistingData AS ec)

Above condition is similar as below condition.

WHERE ID !=1 AND ID !=2 AND ID !=3 AND ID !=NULL AND ID !=4 AND ID !=5