Summary: in this tutorial, you will learn how to use the Db2 UNION to combine the result sets of two or more subselects into a single result set.
Db2 UNION Overview
Db2 UNION operator allows you to combine the result sets of two or more subselects into a single result set.
When Db2 encounters a UNION operator, it carries the following operations:
- First, process each subselect to form an interim result table.
- Then, combine these interim tables and delete the duplicate rows to form the final result set.
Here is the syntax of the Db2 UNION:
1 2 3 4 | subselect_1 UNION subselect_2 |
The queries in the above syntax must meet the following requirements:
- Both subselects must have The number and the order of the columns.
- The data types of the corresponding columns must be the same or compatible via implicit conversion.
The UNION operator is useful to marge lists of values retrieved from multiple tables.
Db2 UNION example
We will use the customers and contacts tables created in the join tutorial for the demonstration.
The following query uses the UNION operator to combine the customers and contacts into a single list. To distinguish customers and contacts, we add a new column named person_type.
1 2 3 4 5 6 7 8 9 10 11 12 13 | SELECT customer_id person_id, name, 'customer' person_type FROM customers UNION SELECT contact_id person_id, name, 'contact' person_type FROM contacts; |
Here is the result set:
Using UNION with ORDER BY clause
When you use the ORDER BY clause in a query that uses the UNION operator:
1) You place the ORDER BY clause after the last subselect:
1 2 3 4 5 6 7 8 9 10 11 | SELECT select_list FROM table_1 UNION SELECT select_list FROM table_2 ORDER BY sort_expression; |
2) You can use the column name in the ORDER BY clause if the column in the list has a name. In case the column of a result set does not have a name, you can use the column alias to assign it a name or use a positive integer in the ORDER BY clause to order the rows.
1 2 3 4 5 6 | SELECT expression AS name FROM table_1 UNION SELECT ... FROM table_2 ORDER BY name; |
or
1 2 3 4 5 6 | SELECT expression FROM table_1 UNION SELECT ... FROM table_2 ORDER BY 1 |
DB2 UNION vs. UNION ALL
The UNION removes all duplicate rows by default. However, if you want to retain the duplicates, you use UNION ALL instead:
1 2 3 | subselect_1 UNION ALL subselect_2 |
Db2 UNION ALL example
The following example uses UNION operator that removes all duplicate rows:
1 2 3 4 5 6 7 8 9 10 11 | SELECT name FROM customers UNION SELECT name FROM contacts ORDER BY name; |
The following picture illustrates the union of two result sets: customers and contacts.
However, the following example uses the UNION ALL operator that retains the duplicate rows:
1 2 3 4 5 6 7 8 9 10 11 | SELECT name FROM customers UNION ALL SELECT name FROM contacts ORDER BY name; |
The following picture illustrates the union all of two result sets: customers and contacts.
UNION vs. JOIN
The join clause combines columns from two or more tables while the UNION operator combines rows from two or more subselects.
In other words, join appends the result sets from tables horizontally while UNION appends result sets from subselects vertically.
In this tutorial, you have learned how to use the Db2 UNION to combine rows from two or more subselects into a single result set.