SQL Server - Difference between Union and Union All

Introduction:

Here I will explain difference between UNION and UNION ALL in SQL server. Union operator will return unique records from tables and Union All in SQL server will return all the records from the tables including duplicate values also.

Description:

In previous articles I explained SQL Server change identity column value,  replace multiple spaces with single space in sql, reseed identity column value in sql server, get only month and year from date in sql server, substring function in SQL server and many articles relating to SQL server. Now I will explain difference between UNION and UNION ALL in SQL server.


Generally Union Operators are used to combine the result of two or more select queries into single result set.

SQL UNION Operator:

SQL Union Operator is used to combine the result of two or more select statement queries into single result set. The Union Operator is used to select only distinct values from two tables.

SQL Union Operator Syntax:

SELECT column1,column2 FROM table1
UNION
SELECT column1,column2 FROM table2
Here one more thing we need to remember that is we can use Union Operator for the tables which is having same column names and same data types otherwise it will throw error like this

All queries combined using a UNION, INTERSECT or EXCEPT operator must have an equal number of expressions in their target lists.
Now I will explain with one example first design two tables in your tables like this

UserInfo

UserID	UserName	Location
1	Suresh	Hyderabad
2	Prasanthi	Hyderabad
3	Mahesh	Vizag

After that create another table and give name as UserDetails

UserID	UserName	Location
1	Suresh	Hyderabad
2	Nagaraju	Bangalore
3	Madhav	Nagpur

Now write the Union Operator Query to get all the user details from two tables like this

SELECT UserName,Location FROM UserInfo
UNION
SELECT UserName,Location FROM UserDetails
Resultant table will be like this

UserName	Location
Suresh	Hyderabad
Prasanthi	Hyderabad
Mahesh	Vizag
Nagaraju	Bangalore
Madhav	Nagpur

If you observe above resultant table it contains UserDetails with distinct records because Union Operator will return only distinct records. If we want all the records then we need to use UNION ALL Operator.

SQL UNION ALL Operator:

This operator is used in a situation like return all the records from the tables including duplicate values also.

SQL UNION ALL Operator Syntax:

SELECT column1,column2 FROM table1
UNION ALL
SELECT column1,column2 FROM table2
Result table

UserName	Location
Suresh	Hyderabad
Suresh	Hyderabad
Prasanthi	Hyderabad
Mahesh	Vizag
Nagaraju	Bangalore
Madhav	Nagpur

The main difference between Union and Union ALL operator is

Union operator will return distinct values but Union ALL returns all the values including duplicate values.