SQL Union Operator | SQL Union ALL Operator | Difference between Union and Union ALL Operators in SQL |

Introduction:

Here I will explain about SQL UNION and UNION ALL operators and differences between UNION and UNION ALL in SQL Server

Description

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.