Union in SQL

The SQL UNION is used to combine the results of two or more SELECT SQL statements into a single result. Each query statement should have same column structure: same number of columns, same or compatible data types and in same order. The columns in each SELECT statement must be in exactly the same order too. You have to keep the column order of all unionized SQL SELECT expressions uniform, otherwise you’ll get an error.

SQL UNION Query Syntax

SELECT ColumnName(s) FROM Table1  

UNION  

SELECT ColumnName (s) FROM Table2  

Note: By Default result  of UNION is distinct. If you want all record you have to use UNION ALL

SQL UNION Query Example

Table: TestingEmployee

EmployeeId	FirstName	LastName
203	Rishi	Rawat
204	Michel	Jones
205	Nathen	Astel
206	Monica	George

Table: DevelopmentEmployee

EmployeeId	FirstName	LastName
206	Monica	George
207	Anita	Paul
208	John	Pit
209	Jennifer	Bob

Select all the different employees in TestingEmployee and DevelopmentEmployee:

SELECT EmployeeId, FirstName, LastName FROM TestingEmployee

UNION  

SELECT EmployeeId, FirstName, LastName FROM DevelopmentEmployees  

The result will look like:

EmployeeId	FirstName	LastName
203	Rishi	Rawat
204	Michel	Jones
205	Nathen	Astel
206	Monica	George
207	Anita	Paul
208	John	Pit
209	Jennifer	Bob

Note: Record of EmloyeeID 206 is in both tables but in result it display only one time if we use UNION ALL then the result display duplicate records for EmployeeId 206