MySQL 'isn't in GROUP BY' Error: How to Fix Full Group By Issues?

Group By Error in MySQL: "'isn't in GROUP BY'"

When encountering the "isn't in GROUP BY" error in MySQL, it indicates that you have a discrepancy between the columns selected in the SELECT clause and those included in the GROUP BY clause.

Understanding the Error

In the given examples, the first query (SELECT count(DISTINCT name)) correctly groups by a single column (name), while the second query (SELECT name, type, language, code`) attempts to retrieve more columns than are included in the GROUP BY` clause.

Resolution

To resolve this error, it's essential to ensure a "full group by," where all columns selected must also be part of the GROUP BY clause. This ensures that the result represents a correct summary for each group.

For the given query, the correct syntax would be:

SELECT `name`, `type`, `language`, `code` 
FROM `users` 
WHERE `verified` = '1' 
GROUP BY `name`, `type`, `language`, `code` 
ORDER BY `count` DESC LIMIT 0, 25

SQL Standards and MySQL Behavior

SQL92 requires that all columns in the SELECT clause be part of the GROUP BY clause. SQL99 relaxes this restriction slightly, allowing columns that are functionally dependent on the GROUP BY clause.

MySQL's default behavior allows for "partial group by," which means that columns not included in the GROUP BY clause may be selected, but this can result in non-deterministic results. To prevent this, you can set the @@sql_mode variable to 'ONLY_FULL_GROUP_BY'. This will force MySQL to adhere to the full group by requirement.

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