Will {fn concat} work without issue in MySQL, SQL Server and Oracle?
This seems to work only through a JDBC connection. Your SELECT works with Oracle, PostgreSQL and DB2 when using a Java/JDBC based query tool. 

Using the native tools this statement throws an error:

Connected to:
Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - Production
With the Partitioning and OLAP options

SQL&gt; SELECT  {fn concat ('foo', 'bar')} from dual;
SELECT  {fn concat ('foo', 'bar')} from dual
        *
ERROR at line 1:
ORA-00911: invalid character


The same is true for PostgreSQL:

psql (9.1.1)
Type "help" for help.

postgres=&gt; SELECT  {fn concat ('foo', 'bar')};
ERROR:  syntax error at or near "{"
LINE 1: SELECT  {fn concat ('foo', 'bar')};


and DB2:

(c) Copyright IBM Corporation 1993,2007
Command Line Processor for DB2 Client 9.7.0

db2 =&gt; SELECT  {fn concat ('foo', 'bar')} from sysibm.sysdummy1;
SQL0104N  An unexpected token "SELECT  {" was found following
"BEGIN-OF-STATEMENT".  Expected tokens may include:  "&lt;values&gt;".
SQLSTATE=42601