In Oracle, when you concatenate a string with a NULL, you get the original string.
Example:
SQL> select title,firstname,
2 title || firstname
3 from employee;
TITL FIRSTNAME TITLE||FIRSTNAME
---- ------------------------- -----------------------------
Ed Ed
But, when doing the same thing on DB2/400, I found that concatenating anything to NULL returns NULL.
Example:
> select title,firstname,
title || firstname
from employee;
title firstName TITLE || FIRSTNAME
- Ed -
Therefore, Oracle acts differently than the iSeries on concatenating NULLs. On the iSeries, if either operand is NULL, the result is NULL. On Oracle, if either operand is NULL, the result is just the other operand. On the iSeries, we can use COALESCE (with an empty string) to produce the same result as Oracle. Example: > select title,firstname, coalesce(title,'') || firstname concatted from employee title firstName CONCATTED - Ed Ed --------------------------------------------------------------------According to the IBM SQL Reference manual: Chapter 2 -- Language Elements -- Expressions: With the Concatenation Operator "If either operand can be null, the result can be null, and if either is null, the result is the null value. Otherwise, the result consists of the first operand string followed by the second."
This was first published in July 2005