The UNION Operator
The UNION Operator is used to combine the result sets of two or more queries into a single result set.
The data type of each column must be the same.
The Order of the columns must be the same in each select statement.
SYNTAX:
            SELECT column1, column2 , column3 , ..
            FROM table1
            UNION
            SELECT column1, column2 , column3, …                                                                      FROM table2;
Let’s understand the union with an example. For demonstration purposes, we will be using the table emp.
UNION
Example:
  Objective: In this example, we will combine the results set of the employee table and the customer table.
           SELECT Ecode, FirstName, LastName, Emailaddress FROM Tbl_Emp                                               UNION                                                                                                SELECT Ecode, FirstName, LastName, Emailaddress FROM Tbl_Customer          Â
Example:
  Objective: In this example, we will combine the results set of the employee table and the customer table with conditions.
           SELECT Ecode, FirstName, LastName, Emailaddress FROM Tbl_Emp  WHERE Department = ‘IT’                        UNION                                                                                                SELECT Ecode, FirstName, LastName, Emailaddress FROM Tbl_Customer WHERE  Department = ‘IT’        Â
UNION ALL
Example:
  Objective: In this example, we will combine the results set of the employee table and the customer table with conditions.
           SELECT Ecode, FirstName, LastName, Emailaddress FROM Tbl_Emp  WHERE Department = ‘IT’                        UNION ALL                                                                                            SELECT Ecode, FirstName, LastName, Emailaddress FROM Tbl_Customer WHERE  Department = ‘IT’        Â
UNION vs UNION ALL
UNION will retrieve unique records from two or more queries.
UNION ALL will retrieve duplicate records from two or more queries.