Package expressions are a feature that allows simple expressions to be used in various parts of a package. They are a very important feature of Integration Services because they simplify packages by facilitating surgical, intelligent, dynamic, and subtle adjustments to the flow and settings of the package. Understanding how to use expressions is critical to creating effective packages and getting the most from Integration Services. This chapter is a gentle introduction to package expressions. Appendix B, "Expression Syntax and Cookbook," covers more detailed descriptions of expression evaluator functions and syntax. The expression syntax is a combination of operators and math, string, and T-SQL like functions. See Appendix B for a listing of the expression functions and operators. Expressions can use package variables and can perform useful functions, such as concatenating strings, calculating dates, building unique filenames, or generating unique numbers. Package expressions can be found on virtually every object in Integration Services, but how they are used and when they are evaluated is different depending on where the expression is found. There are several types of package expressions, as follows:
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