The union clause places two serarate queries together forming one table. A union works best when using two tables with similar columns because each cloumn must have the same data type.
select field1, field2, . field_n from tables UNION select field1, field2, . field_n from tables;UNION ALL
The UNION ALL query allows you to combine the result sets of 2 or more "select" queries. It returns all rows (even if the row exists in more than one of the "select" statements).
Each SQL statement within the UNION ALL query must have the same number of fields in the result sets with similar data types.
The syntax for a UNION ALL query is:
select field1, field2, . field_n from tables UNION ALL select field1, field2, . field_n from tables;
select supplier_id from suppliers UNION ALL select supplier_id from orders;
