SQL: UNION ALL Query
The SQL UNION ALL query allows you to combine the result sets of 2 or more SELECT statements. It returns all rows from the query (even if the row exists in more than one of the SELECT statements).
Each SQL SELECT statement within the SQL UNION ALL query must have the same number of fields in the result sets with similar data types.
The syntax for the SQL UNION ALL query is:
select field1, field2, ... field_n from tables UNION ALL select field1, field2, ... field_n from tables;
SQL UNION ALL Query - Returns single field example
The following is an example of the SQL UNION ALL query that returns one field from multiple SELECT statements (and both fields have the same data type):
select supplier_id from suppliers UNION ALL select supplier_id from orders;
This SQL UNION ALL query would return a supplier_id multiple times in your result set if the supplier_id appeared in both the suppliers and orders table. The SQL UNION ALL query does not remove duplicates. If you wish to remove duplicates, try using the SQL UNION query.
SQL UNION ALL Query - Using SQL ORDER BY Clause example
The SQL UNION ALL query can use the SQL ORDER BY clause to order the results of the query.
select supplier_id, supplier_name from suppliers where supplier_id > 2000 UNION ALL select company_id, company_name from companies where company_id > 1000 ORDER BY 2;
In this SQL UNION ALL query, since the column names are different between the two SQL SELECT statements, it is more advantageous to reference the columns in the SQL ORDER BY clause by their position in the result set. In this example, we've sorted the results by supplier_name / company_name in ascending order, as denoted by the "ORDER BY 2".
The supplier_name / company_name fields are in position #2 in the result set.