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Description

SQL Server Error 518 indicates a failed attempt to convert data from one data type to another. This error typically arises during data insertion, updates, or when performing operations that implicitly convert data types.

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Error Message

Cannot convert data type %ls to %ls.

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Known Causes

3 known causes

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Implicit Conversion Failure

SQL Server attempts to automatically convert data types, but the conversion is invalid or unsupported, leading to the error. This often happens when inserting a string into a numeric column.

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Explicit Conversion Error

Using `CAST` or `CONVERT` functions to explicitly change data types, but the specified conversion is not valid. For example, trying to convert a very long string into a small integer.

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Incorrect Data Import

Importing data from external sources (e.g., CSV files) with mismatched data types compared to the SQL Server table definition. This is common when the external source has a different data type than the destination column.

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Solutions

4 solutions available

1. Explicitly Cast or Convert Data Types easy

Manually convert data types within your query to match expected formats.

1

Identify the columns and the target data type involved in the conversion error. This information is usually provided in the error message itself (e.g., '%ls' to '%ls').

2

Use the `CAST()` or `CONVERT()` function in your SQL query to explicitly convert the problematic column to the desired data type. Choose the function based on your preference or specific conversion needs (e.g., `CONVERT` offers more style options).

SELECT CAST(ColumnToConvert AS TargetDataType) AS ConvertedColumn FROM YourTable;
-- or
SELECT CONVERT(TargetDataType, ColumnToConvert) AS ConvertedColumn FROM YourTable;

3

If you are inserting or updating data, ensure the source data can be successfully converted to the destination column's data type. You might need to perform the conversion before the insert/update statement.

INSERT INTO TargetTable (Column1, Column2)
SELECT CAST(SourceColumn1 AS TargetDataType1), SourceColumn2
FROM SourceTable;

2. Modify Table Schema to Align Data Types medium

Change the data type of a table column to accommodate the incoming data.

1

Determine which table and column are causing the data type mismatch. This often occurs during data loading or when inserting/updating data from another source.

2

Use the `ALTER TABLE` statement to modify the data type of the column. Ensure the new data type can accommodate the existing and incoming data. If the column contains incompatible data, you may need to clean it first.

ALTER TABLE YourTable
ALTER COLUMN ColumnToModify NewDataType;

3

Before altering the column, consider backing up the table or relevant data, especially if the column contains critical information or if the data type change is significant.

3. Cleanse Source Data Before Insertion/Update medium

Pre-process data to ensure it conforms to the target data type's requirements.

1

Identify the source of the data that is causing the conversion error. This could be an external file, another table, or a result of a previous query.

2

Implement data cleansing logic to handle or remove values that cannot be converted. This might involve using conditional logic, string manipulation, or error handling.

SELECT 
    CASE 
        WHEN ISNUMERIC(ColumnToConvert) = 1 THEN CAST(ColumnToConvert AS TargetDataType)
        ELSE NULL -- or some default value
    END AS CleanedColumn
FROM SourceTable;

3

If loading data from files (e.g., CSV, Excel), use data integration tools (like SQL Server Integration Services - SSIS) or scripting (PowerShell, Python) to perform data profiling and cleansing before loading into SQL Server.

4. Review Application Code or ETL Processes advanced

Examine the code that is performing the data operation for incorrect data type handling.

1

Trace the data flow from your application or ETL process to the point where the SQL Server error occurs. Identify the specific operation (INSERT, UPDATE, SELECT) and the data being processed.

2

In your application code (e.g., C#, Java, Python), ensure that you are correctly mapping and converting data types before sending them to SQL Server. Many database connectors provide methods for type conversion.

C# example:
var command = new SqlCommand("INSERT INTO YourTable (NumericColumn) VALUES (@Value)", connection);
command.Parameters.AddWithValue("@Value", Convert.ToDecimal(stringValue));

3

If using an ETL tool like SSIS, review the data flow tasks, transformations, and derived column components. Ensure that any data type conversions within the ETL package are correctly configured and that the source data is validated.

SQL Server: Data Conversion