Application Programming and SQL Guide - Kmlinux

Merging lists of values: UNION
Using the UNION keyword, you can combine two or more SELECT statements to
form a single result table. When DB2 encounters the UNION keyword, it processes
each SELECT statement to form an interim result table, and then combines the
interim result table of each statement. If you use UNION to combine two columns
with the same name, the result table inherits that name.
When you use the UNION statement, the SQLNAME field of the SQLDA contains
the column names of the first operand.
Using UNION to eliminate duplicates
You can use UNION to eliminate duplicates when merging lists of values obtained
from several tables.
Example: UNION clause: You can obtain a combined list of employee numbers
that includes both of the following:
v People in department D11
v People whose assignments include projects MA2112, MA2113, and AD3111.
The following SQL statement gives a combined result table containing employee
numbers in ascending order with no duplicates listed:
SELECT EMPNO
FROM DSN8810.EMP
WHERE WORKDEPT = ’D11’
UNION
SELECT EMPNO
FROM DSN8810.EMPPROJACT
WHERE PROJNO = ’MA2112’ OR
PROJNO = ’MA2113’ OR
PROJNO = ’AD3111’
ORDER BY EMPNO;
If you have an ORDER BY clause, it must appear after the last SELECT statement
that is part of the union. In this example, the first column of the final result table
determines the final order of the rows.
Using UNION ALL to keep duplicates
If you want to keep duplicates in the final result table of a UNION, specify the
optional keyword ALL after the UNION keyword.
Example: UNION ALL clause: The following SQL statement gives a combined
result table containing employee numbers in ascending order, and includes
duplicate numbers:
SELECT EMPNO
FROM DSN8810.EMP
WHERE WORKDEPT = ’D11’
UNION ALL
SELECT EMPNO
FROM DSN8810.EMPPROJACT
WHERE PROJNO = ’MA2112’ OR
PROJNO = ’MA2113’ OR
PROJNO = ’AD3111’
ORDER BY EMPNO;
Chapter 1. Retrieving data 13