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Description

This error indicates that the `GROUP_CONCAT()` function truncated data from one or more rows because the resulting string exceeded its maximum allowed length. It typically occurs when the combined length of concatenated values in a group surpasses the `group_concat_max_len` system variable.

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Error Message

Row %u was cut by GROUP_CONCAT()

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Known Causes

4 known causes

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Insufficient `group_concat_max_len`

The `group_concat_max_len` system variable sets the byte limit for `GROUP_CONCAT()` results. A value that is too low will cause truncation.

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Large Data Volumes

Concatenating many rows or very long strings within a group can quickly exceed the `group_concat_max_len` limit, even if it's set to a default value.

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Multi-byte Character Sets

When using character sets like UTF-8, each character can consume multiple bytes. This reduces the effective character count before the `group_concat_max_len` byte limit is reached.

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Inefficient Query Logic

Query designs that aggregate an unexpectedly large number of items or very long strings using `GROUP_CONCAT()` can easily trigger this limitation.

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Solutions

3 solutions available

1. Increase GROUP_CONCAT Length easy

Temporarily or permanently increase the maximum length for GROUP_CONCAT.

1

To temporarily increase the `group_concat_max_len` for the current session, execute the following SQL command:

SET SESSION group_concat_max_len = 1024000;

2

Replace `1024000` with a value large enough to accommodate your expected concatenated string length (in bytes). A common starting point is 1MB (1024 * 1024). You can also set it to a larger value like `@@max_allowed_packet` if you have it configured high.

SET SESSION group_concat_max_len = @@max_allowed_packet;

3

To permanently increase the `group_concat_max_len` for all connections, you need to modify the MariaDB configuration file. Locate your `my.cnf` or `my.ini` file (the location varies by OS and installation). Add or modify the following line under the `[mysqld]` section:

[mysqld]
group_concat_max_len = 1024000

4

After modifying the configuration file, restart the MariaDB service for the changes to take effect.

# On systemd-based systems:
sudo systemctl restart mariadb

# On older sysvinit systems:
sudo service mariadb restart

2. Adjust Data Types or Use a Different Approach medium

Consider if GROUP_CONCAT is the right tool or if data types need adjustment.

1

Review the data being concatenated. If it's very large text or binary data, `GROUP_CONCAT` might not be the most efficient or appropriate function. Consider if you can truncate or summarize the data before concatenation.

SELECT LEFT(long_text_column, 255) AS truncated_text FROM your_table;

2

If you are concatenating multiple, potentially large, string columns, consider if it's necessary to include all of them. You might be able to select a subset or use a different string aggregation function if available (though `GROUP_CONCAT` is standard).

SELECT GROUP_CONCAT(short_column_1, short_column_2 SEPARATOR ', ') FROM your_table;

3

If the error occurs with binary data (like BLOBs), `GROUP_CONCAT` is generally not suitable. You should handle binary data differently, perhaps by storing paths to files or using other mechanisms to retrieve and process it.

3. Paginate or Process in Chunks medium

Break down the query to avoid overwhelming GROUP_CONCAT.

1

If you have a very large number of rows that would result in an extremely long concatenated string, consider fetching data in smaller batches or chunks. This might involve adding a `LIMIT` and `OFFSET` clause or using a cursor if your application logic allows.

SELECT GROUP_CONCAT(column_to_concat SEPARATOR ',') FROM your_table WHERE some_condition LIMIT 1000 OFFSET 0;

2

Execute subsequent queries with increasing `OFFSET` values to retrieve all data progressively. This requires application-level logic to manage the pagination.

SELECT GROUP_CONCAT(column_to_concat SEPARATOR ',') FROM your_table WHERE some_condition LIMIT 1000 OFFSET 1000;

3

Alternatively, if you are processing this in an application, fetch rows in batches and perform the concatenation within your application code. This gives you more control over memory usage.

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Related Errors

5 related errors

MariaDB Error 1260: GROUP_CONCAT() Data Truncation