ora-01858: a non-numeric character was found where a numeric was expected

3 min read 01-10-2024

ora-01858: a non-numeric character was found where a numeric was expected

The Oracle error code ORA-01858 is a common issue encountered by developers and database administrators working with date formats in Oracle databases. This error message typically indicates that there is a mismatch between the expected format of a date and the actual data provided. Let's delve deeper into the causes of this error, how to resolve it, and some best practices to prevent it.

What is ORA-01858?

The error message reads as follows:

ORA-01858: a non-numeric character was found where a numeric was expected

This error occurs when Oracle encounters a non-numeric character (such as a letter or special character) in a string that is expected to be numeric, particularly during date conversions or when parsing date literals.

Common Causes

Incorrect Date Format: One of the most frequent causes of this error is when the format of the date string does not match the expected format specified in the TO_DATE or TO_CHAR functions. For example, if you are trying to convert a date string in the format of DD/MM/YYYY but using a format mask like MM/DD/YYYY, it will cause this error.
Unexpected Characters: If a date string contains unexpected characters, such as whitespace, letters, or symbols where numbers are expected, the conversion will fail.
Database NLS Settings: The Oracle database NLS (National Language Support) settings may affect how date formats are interpreted, which can lead to this error if the input does not align with the configured settings.

Example Scenario

Consider the following SQL query:

SELECT TO_DATE('2023-01-30', 'YYYY-MM-DD') FROM dual;

If the date string passed is supposed to adhere to a different format and the actual input does not match this, you might receive the ORA-01858 error.

How to Resolve ORA-01858

Check the Format: Ensure that the format of the date string matches the expected format in the TO_DATE function. If your input is 30/01/2023 but you are using YYYY-MM-DD, you would see an error. Adjust the format accordingly:
```
SELECT TO_DATE('30/01/2023', 'DD/MM/YYYY') FROM dual;
```
Sanitize Input Data: Before attempting to convert or parse dates, ensure that the input data is cleaned up. Remove any leading or trailing spaces and validate that the input matches the expected numeric format.
Adjust NLS Settings: If you suspect that NLS settings might be affecting date formats, you can check and modify them as necessary:
```
SELECT * FROM NLS_SESSION_PARAMETERS WHERE PARAMETER LIKE 'NLS_DATE_FORMAT';
```
To change the NLS date format, you might execute:
```
ALTER SESSION SET NLS_DATE_FORMAT = 'DD/MM/YYYY';
```

Practical Example of ORA-01858 Resolution

Suppose you have a table employees with a column hire_date stored as VARCHAR2 but formatted inconsistently. If you want to convert and retrieve dates:

SELECT employee_name,
       TO_DATE(hire_date, 'DD/MM/YYYY') AS formatted_hire_date
FROM employees;

If the hire_date contains strings like 02/30/2023 or abc/12/2023, you will encounter the ORA-01858 error. Before running this query, you could filter out invalid entries with a regex pattern:

SELECT employee_name,
       TO_DATE(hire_date, 'DD/MM/YYYY') AS formatted_hire_date
FROM employees
WHERE REGEXP_LIKE(hire_date, '^\d{2}/\d{2}/\d{4}{{content}}#39;);

Best Practices to Avoid ORA-01858

Always Validate Input: Implement data validation before inserting or processing dates to ensure they conform to expected formats.
Use Consistent Date Formats: Stick to a uniform date format across your applications and databases to minimize confusion.
Handle Exceptions Gracefully: Use error handling in your SQL queries or application logic to catch and log ORA-01858 errors, providing meaningful feedback to users or developers.

Conclusion

The ORA-01858 error can be a frustrating hurdle in date handling with Oracle databases, but understanding its causes and how to resolve it can save developers and DBAs significant time and effort. By adhering to best practices in date validation and ensuring consistent use of formats, you can avoid this error in your future database interactions.

For further details and discussions, you can refer to the original thread on Stack Overflow. Always make sure to consult the Oracle documentation and community for updated information and tips.

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