An SSIS expression combines columns, functions, operators, and literals to produce values for new or replaced columns in a data flow. A common mistake is treating staging values as already compatible: conversions may require explicit casts, formatting changes, or NULL handling. section 6.3.2
Use the Derived Column transformation to build and check expressions:
- Name the output column, choose whether to replace an existing column or add a new one, and enter the expression freeform or by dragging columns, variables, parameters, and functions. section 6.3.2
- Treat a red expression as a parse failure; hover over it to see the diagnostic details before running the data flow. section 6.3.2
- Check the automatically populated data type, length, precision, scale, and code page after conversion. section 6.3.2
Converting staged marketing data
Convert a GUID by casting it to a 36-character Unicode string, adding curly braces, and casting to DT_GUID: (DT_GUID)("{" + (DT_WSTR,36)(ImpressionUID) + "}"). Convert EventTime by replacing slashes with hyphens before casting to DT_DBTIMESTAMP. Use a conditional expression to turn the string "NULL" into a typed NULL. section 6.3.2
