Expression Language Patterns - SQL Server Integration Services Design Patterns - page 284

have to write by relying on the built-in string manipulation functions
in the expression language.
Maintenance scope : By applying the design pattern of using the ex-
pression language for cleansing needs, you can eliminate much of the
sleuth work you need to do to track down and change cleansing rules
as your business expectations change. Using expressions in the pack-
age itself provides you with a single point of maintenance rather than
forcing you to inspect the upstream data sources each time you need
to make a change.
I've done a number of presentations for novice SSIS developers, and when I bring
up the topic of the expression language, one question almost always seems to come up:
“Where do I use this expression language stuff?” My answer: “Everywhere!” Part of
the beauty of expressions is that they can be used almost anywhere within SSIS pack-
ages. You can employ expressions on the control flow in precedence constraints. It's
convenient to make your SSIS package variables dynamic by replacing their static val-
ues with expressions. You can leverage expressions within the data flow to manipulate
data and even control the execution path. The bottom line is that you can manipulate
many of the common properties of packages, tasks, constraints, and data flow elements
by using expressions.
Although its syntax may seem unusual, the expression language isn't difficult to
learn. Anyone with logical scripting experience (even if that experience is limited to T-
SQL) can quickly pick up on the basics and should be able to master the language with
a reasonable amount of practice.
Language Essentials
Even for those who have experience scripting in other Microsoft development environ-
ments, the first exposure to the SSIS expression language can be a little unsettling. The
syntax and functionality are unlike any other language, either interpreted or compiled.
It appears to be a strange hybrid of several languages and is certainly a dialect all of its
own.
Developers who have spent time using the C-style languages (C, C++, C#, Java)
will recognize some of the syntactical nuances within the expression language:
• Case-sensitive column and variable names
• Case-sensitive string comparisons