Two tables, or two query results, can be combined into a single result table with UNION.
Both SELECT statements must return the same number of columns with compatible data types.
The column names of the combined result come from the first SELECT.
UNION vs UNION ALL
UNION removes duplicate rows. Removing duplicates needs a sort, so it is slower.
UNION ALL keeps duplicate rows. No sort is needed, so it is faster.
Choose UNION ALL when duplicates are impossible or when you want to keep them.
Example:-
-- Combined list, duplicates removed
SELECT EMPNO, FNAME FROM EMPS_A
UNION
SELECT EMPNO, FNAME FROM EMPS_B;
-- Combined list, duplicates kept, runs faster
SELECT EMPNO, FNAME FROM EMPS_A
UNION ALL
SELECT EMPNO, FNAME FROM EMPS_B;
Mainframe Tutorial
JCL Tutorial
Introduction
Mainframe Interview Q&A
Mainframe Quiz
Mainframe Memes
Contact us
About us
Backward
