Expression Language Patterns - SQL Server Integration Services Design Patterns - page 293

ample, you can include a relatively simple expression to bypass the execution of the
majority of the package when no files are found to process. Saving those steps saves
CPU cycles, disk I/O, and other resources.
Precedence constraints also have the ability to use expressions to enforce proper
package flow. In Figure 10-4 , you'll see that the evaluation operation is set to Expres-
sion to enforce both the execution value of the prior task as well as the value defined in
the Expression box. For illustration purposes, assume that you've populated an SSIS
variable to store the number of files downloaded in the Script task operation, and
you're using the expression to confirm that at least one file was processed. From here,
you can either type the expression into the window manually or use the ellipsis button
to open the Expression Editor (note that in earlier versions (2008 and earlier) of the
product, you will have to enter the expression by hand without the benefit of the Ex-
pression Editor).
Figure 10-4 . Precedence Constraint Editor
Refer back to the original package; you'll see that the precedence constraint
between the first Script task and the truncation SQL task now reflects the presence of
an expression in the constraint ( Figure 10-5 ) .