SQL Union

The SQL UNION clause can be used to concatenate the results of multiple SELECT statements together into a single result.

In the general form, it is used as linkage between two or more SELECT statements as below:

SELECT … FROM …  
UNION
SELECT … FROM …  
UNION
SELECT … FROM …  
…

Since the results are being concatenated, it is a requirement that the schema (number of columns and data types) in the results of the individual queries match, otherwise the results will not be able to be concatenated.

For example, we can put together the results of two simple SELECT statements from the employees table using USING, to find employees who have first names either “Georgi” or “Annelise”:

SELECT * FROM employees WHERE first_name = "Georgi"
UNION
SELECT * FROM employees WHERE first_name = "Anneke";

For this simple example, it should be noted that an equivalent result can be achieved using a WHERE clause and OR with the two predicates, or using IN and a single predicate with both names:

SELECT * FROM employees WHERE first_name = "Georgi" OR first_name = "Anneke";

In practice, UNION is most often used when combining different tables with the same schema or in more complex data engineering SQL code. For simpler queries coming from the same table, an equivalent result can usually be arrived upon without requiring its usage, as seen in above.