MySQL SHOW WARNINGS Statements
The `SHOW WARNINGS` statement in MySQL is used to display warnings, errors, and notes generated by the last SQL statement executed. It provides insights into potential issues or information about the execution process that may not halt the execution but may require attention.
Usage
`SHOW WARNINGS` is particularly useful after executing SQL statements that can generate warnings, such as data import operations, `INSERT`, `UPDATE`, or `DELETE` statements. It helps identify non-critical issues that need addressing to ensure data integrity or performance optimization. If no preceding SQL statement is executed, `SHOW WARNINGS` will return an empty result.
SHOW WARNINGS;
This syntax retrieves a list of warnings, errors, and notes for the most recently executed SQL statement.
Examples
1. Basic Usage
SHOW WARNINGS;
After executing a statement, this command displays all warnings related to the last SQL operation. Note that `SHOW WARNINGS` displays warnings for the current session only.
2. Viewing Specific Number of Warnings
SHOW WARNINGS LIMIT 5;
This example shows how to limit the output to the first five warnings, useful for large sets of warnings.
3. Detailed Warnings Information
SHOW WARNINGS;
SELECT @@warning_count AS 'Total Warnings';
Here, `SHOW WARNINGS` provides the details, and the subsequent query returns the total number of warnings.
Tips and Best Practices
- Regularly check warnings. Use `SHOW WARNINGS` after executing complex queries to catch potential issues early.
- Limit output for large data sets. When dealing with many warnings, use `LIMIT` to focus on the most critical or first few warnings.
- Automate checks in scripts. Integrate `SHOW WARNINGS` into scripts to automate the review process and ensure consistent monitoring.
- Combine with `@@warning_count`. Use `@@warning_count` to programmatically assess the number of warnings and take conditional actions in scripts.
- Understand session-specific behavior. Remember that `SHOW WARNINGS` results are session-specific, ensuring that you are viewing warnings from your current work.
- Consider client settings. Be aware that client settings, such as `sql_mode`, can affect the warnings generated, influencing what `SHOW WARNINGS` displays.
- Mind the `max_error_count`. The output of `SHOW WARNINGS` is limited by the `max_error_count` system variable, relevant for extensive warnings.
- Interpret output columns. The `SHOW WARNINGS` output includes columns: `Level` (indicating the severity), `Code` (the error code), and `Message` (detailed message), which help in understanding the context and nature of each warning.