Select Page

Replacing UNION with CASE

Craig Mullins | | November 22, 2017

dba db2

When a UNION is required to put together data from multiple queries, you might be able to use a CASE statement instead. This is very useful, particularly when the data for each of the queries in the UNION come from the same table. The CASE statement can potentially enhance performance by minimizing the number of times the data is read.

Let’s look at an example to clarify why:

SELECT CREATOR, NAME, ‘TABLE’
FROM SYSIBM.SYSTABLES
WHERE TYPE = ‘T’
UNION
SELECT CREATOR, NAME, ‘VIEW ‘
FROM SYSIBM.SYSTABLES
WHERE TYPE = ‘V’
UNION
SELECT CREATOR, NAME, ‘ALIAS’
FROM SYSIBM.SYSTABLES
WHERE TYPE = ‘A’
ORDER BY NAME;

This simple SQL statement uses UNION to put together the results of three queries against the SYSTABLES table. The report shows all of the DB2 table-like objects that exist in the DB2 subsystem: tables, views, and synonyms.

To do this, DB2 must scan through the table three times – once for each query (as there is no index on the TYPE column). But, you can use CASE and code an equivalent, but more efficient query, as follows:

SELECT CREATOR, NAME,
CASE TYPE
WHEN ‘T’ THEN ‘TABLE’
WHEN ‘V’ THEN ‘VIEW ‘
WHEN ‘A’ THEN ‘ALIAS’
END
FROM SYSIBM.SYSTABLES
ORDER BY NAME;

This new query will need to scan SYSTABLES only once. The CASE statement will translate the code in the TYPE column into the text that we desire.

CASE statements are very powerful and you should use them when you can to create elegant and optimal SQL in your DB2 applications.

This post was originally published on Craig Mullins’ blog at: http://db2portal.blogspot.com/2009/11/replacing-union-with-case.html

For additional resources please download my white paper: “The Many Different Types of DBAs”

 

12c Upgrade Bug with SQL Tuning Advisor

This blog post outlines steps to take on Oracle upgrade 11.2 to 12.1 if you’re having performance problems. Oracle offers a patch and work around to BUG 20540751.

Megan Elphingstone | March 22, 2017

Oracle EPM Cloud Vs. On-Premises: What’s the Difference?

EPM applications help measure the business performance. This post will help you choose the best EPM solutions for your organization’s needs and objectives.

Bobby Ellis | April 10, 2018
code

Recover a Table from an RMAN Backup in an Oracle 12c

This blog post will is to show a table restore for one table in a container database

Megan Elphingstone | February 2, 2017

Work with Us

Let’s have a conversation about what you need to succeed and how we can help get you there.

CONTACT US

Work for Us

Where do you want to take your career? Explore exciting opportunities to join our team.

EXPLORE JOBS