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Description

This error indicates that MySQL encountered data that is not properly formatted JSON when attempting to convert it to a JSON data type using a `CAST` operation. It typically occurs during `INSERT`, `UPDATE`, or `SELECT` statements where a column's value is explicitly cast to JSON, but the source data violates JSON syntax rules at the specified column and row.

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Error Message

Invalid JSON value for CAST to %s%s from column %s at row %ld

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Known Causes

4 known causes

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Malformed JSON String

The string value in the source column does not adhere to valid JSON syntax, such as missing quotes, incorrect brackets, or unescaped special characters.

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Incorrect Source Data Type

The column being cast contains a non-string data type (e.g., integer, date) that cannot be directly interpreted as a JSON string for casting.

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Empty or NULL String Misinterpretation

An empty string or NULL value is being cast to JSON, and the context expects a specific JSON structure, leading to an invalid conversion.

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Character Encoding Mismatch

Special characters within the source string are not correctly encoded, causing MySQL's JSON parser to fail during syntax validation.

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Solutions

4 solutions available

1. Validate and Sanitize Input Data medium

Ensure data being inserted into JSON columns conforms to valid JSON format.

1

Identify the problematic data. The error message 'from column %s at row %ld' provides clues. Examine the data in the specified column and row.

2

Use a JSON validator to check if the string intended for a JSON column is valid JSON. Online validators or programming language libraries can be used.

3

Cleanse the data before insertion. Remove any invalid characters, ensure proper escaping of special characters (e.g., quotes, backslashes), and verify that the structure is correct (e.g., matching braces, valid key-value pairs).

Example of sanitizing in application code (conceptual):

python
import json

def sanitize_json_string(data_string):
    try:
        # Attempt to parse and re-serialize to ensure validity
        parsed_data = json.loads(data_string)
        return json.dumps(parsed_data)
    except json.JSONDecodeError:
        # Handle the error, e.g., log it, return an error message, or try to fix common issues
        print(f"Invalid JSON detected: {data_string}")
        # Basic fix: try to enclose unquoted strings in quotes if they look like keys
        # This is a simplistic example and might need more robust logic.
        return data_string # Or implement a more sophisticated repair mechanism

# Usage before inserting into MySQL:
# valid_json_string = sanitize_json_string(user_input_string)
# INSERT INTO your_table (json_column) VALUES (valid_json_string);

4

If the data is coming from user input or an external source, implement robust input validation at the application level to prevent invalid JSON from reaching the database.

2. Use JSON_VALID() for Pre-validation medium

Proactively check data for JSON validity before attempting to cast or insert.

1

When inserting or updating data, use the `JSON_VALID()` function to check if a string is a valid JSON document.

INSERT INTO your_table (id, json_column)
VALUES (1, JSON_VALID('{"key": "value"}'));

-- Or to check before inserting:
SET @potential_json = '{"invalid": "json"';

IF JSON_VALID(@potential_json) THEN
    INSERT INTO your_table (id, json_column) VALUES (2, @potential_json);
ELSE
    -- Handle the invalid JSON case, e.g., log an error or use a default value
    SELECT 'Invalid JSON provided.';
END IF;

2

In your application logic, wrap your JSON insertion/update statements with checks using `JSON_VALID()` to catch and handle invalid data before it causes a `3156` error.

3. Handle NULL or Empty Strings Appropriately easy

Ensure NULLs or empty strings are treated as valid JSON nulls or empty objects/arrays.

1

If your JSON column can accept `NULL` values, ensure that `NULL` is explicitly inserted when appropriate, rather than an empty string or malformed JSON.

INSERT INTO your_table (id, json_column)
VALUES (3, NULL);

2

If an empty string is intended to represent an empty JSON value (like an empty object `{}` or empty array `[]`), ensure it's correctly cast or handled. MySQL often interprets an empty string as an invalid JSON value.

INSERT INTO your_table (id, json_column)
VALUES (4, '{}'); -- For an empty JSON object

INSERT INTO your_table (id, json_column)
VALUES (5, '[]'); -- For an empty JSON array

3

Be aware that casting an empty string `''` directly to JSON will likely result in an error. Use `{}` or `[]` for empty JSON structures.

4. Review CAST Operations for Type Mismatches medium

Verify that the data type being cast to JSON is compatible.

1

Examine `CAST()` operations in your SQL queries. The error message `CAST to %s%s` indicates the target JSON type. Ensure the source column's data can be meaningfully converted.

SELECT CAST(your_string_column AS JSON) FROM your_table;

-- If 'your_string_column' contains data like 'hello world' (unquoted) or '{invalid json', this will fail.

2

If you are casting a string column that might contain non-JSON data, use `JSON_VALID()` first, or apply sanitization as described in Solution 1. Alternatively, use `JSON_UNQUOTE()` if you are expecting a JSON string that has been unnecessarily quoted.

SELECT JSON_UNQUOTE(your_column) FROM your_table WHERE JSON_VALID(your_column);

-- Or, more cautiously, handle potential errors:
SELECT
    CASE
        WHEN JSON_VALID(your_column) THEN CAST(your_column AS JSON)
        ELSE NULL -- Or some other default/error indicator
    END AS valid_json_data
FROM your_table;

3

Ensure that you are not trying to cast data types that inherently cannot be JSON (e.g., large binary objects) without proper serialization.

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Related Errors

5 related errors

MySQL Error 3156: Invalid JSON Cast Value