SQL UNION / UNION ALL Basic UNION ALL query
Example
CREATE TABLE HR_EMPLOYEES
(
PersonID int,
LastName VARCHAR(30),
FirstName VARCHAR(30),
Position VARCHAR(30)
);
CREATE TABLE FINANCE_EMPLOYEES
(
PersonID INT,
LastName VARCHAR(30),
FirstName VARCHAR(30),
Position VARCHAR(30)
);
Let's say we want to extract the names of all the managers from our departments.
Using a UNION we can get all the employees from both HR and Finance departments, which hold the position of a manager
SELECT
FirstName, LastName
FROM
HR_EMPLOYEES
WHERE
Position = 'manager'
UNION ALL
SELECT
FirstName, LastName
FROM
FINANCE_EMPLOYEES
WHERE
Position = 'manager'
The UNION statement removes duplicate rows from the query results. Since it is possible to have people having the same Name and position in both departments we are using UNION ALL, in order not to remove duplicates.
If you want to use an alias for each output column, you can just put them in the first select statement, as follows:
SELECT
FirstName as 'First Name', LastName as 'Last Name'
FROM
HR_EMPLOYEES
WHERE
Position = 'manager'
UNION ALL
SELECT
FirstName, LastName
FROM
FINANCE_EMPLOYEES
WHERE
Position = 'manager'
