Ssis 858

Caroline Mae

2 min read

·

05-01-2025

1

0

Share

Error 858 in SQL Server Integration Services (SSIS) is a common headache, often manifesting as the cryptic message: "The data types are incompatible in the expression." This seemingly simple error can stem from a variety of underlying issues, making diagnosis and resolution a bit tricky. Let's break down the most frequent causes and how to tackle them.

Common Causes of SSIS Error 858

The root of the problem almost always lies in a mismatch between data types used in your SSIS package. This mismatch can occur in several places:

1. Data Type Mismatches in Derived Columns

This is arguably the most common scenario. You might be attempting a calculation or transformation in a Derived Column transformation, but the data types of the input columns aren't compatible with the operation or the resulting data type. For example, trying to add a string to an integer will inevitably fail.

Example: Trying to concatenate an integer column (Quantity) with a string column (ProductName) without explicit type casting.

2. Data Type Mismatches in Expressions

Similar to Derived Columns, expressions used throughout your SSIS package (e.g., in Precedence Constraints, Conditional Splits) can trigger this error if the data types involved aren't compatible. Incorrect type handling in your expressions can lead to unexpected outcomes and the dreaded 858.

Example: Using a string comparison operator on a numeric column without proper conversion.

3. Data Type Mismatches Between Sources and Destinations

This error can also surface when the data type of a column in your source (e.g., a database table) doesn't match the corresponding column's data type in your destination (e.g., another database table or a flat file). SSIS needs a perfect alignment to move data efficiently and accurately.

Example: A source column with a DATETIME data type being mapped to a VARCHAR column in the destination.

4. Implicit vs. Explicit Conversions

SSIS handles data type conversions, but it's crucial to understand the difference between implicit and explicit conversions. Implicit conversions are automated, but they might not always work as expected, potentially leading to data loss or errors like 858. Explicit conversions, on the other hand, give you precise control over how the conversion is performed, reducing the risk of errors.

Troubleshooting and Solutions

To resolve SSIS Error 858, follow these steps:

1. Identify the location: Pinpoint the exact component (Derived Column, Expression, etc.) causing the error. The SSIS error message should provide some clues.

2. Examine data types: Carefully check the data types of all involved columns and variables. Use the data viewer to inspect the data at different points in your package.

3. Use explicit casting: Employ explicit casting (e.g., (DT_WSTR) for string, (DT_I4) for integer) to convert data types as needed within your expressions and Derived Column transformations. This ensures proper conversion and avoids ambiguous implicit conversions.

4. Data type mapping: Ensure that the data types in your source and destination components are compatible and aligned. Make necessary adjustments in the data flow mappings.

5. Check data cleansing: Before transformations, consider cleansing the data using components like the Data Conversion Transformation to handle null values, inconsistent formats and other potential issues causing type mismatches.

By systematically investigating the data types and using explicit casting when necessary, you can effectively diagnose and resolve the often frustrating SSIS Error 858. Remember, meticulous attention to data type compatibility is key to a smoothly functioning SSIS package.