Why is MySQL's GROUP BY Behavior Unpredictable Without Aggregate Functions?

Non-deterministic behavior of MySQL GROUP BY clause without aggregate function

When you use the GROUP BY clause in a MySQL query without an aggregate function in the SELECT clause, its behavior may be non-deterministic. This is because MySQL allows columns to be omitted from the GROUP BY clause for performance reasons.

However, this omission only works if the omitted column has the same value in all rows within each grouping. If this is not the case, as shown in the example, the server is free to choose any value in the group. As a result, the return value is undefined.

To ensure predictable results, it is recommended to include all columns that are not aggregated in the GROUP BY clause. Additionally, it is generally not recommended to use the * wildcard character to select all columns in a table, as it can lead to unpredictable behavior in similar situations.

The MySQL documentation clearly states: "When using this feature, all rows in each group should have the same value for the columns omitted from the GROUP BY section." If this condition is not met, the query results are undefined of.

Therefore, be sure to carefully consider the columns included in the GROUP BY clause and avoid omitting columns that contain different values ​​within the grouping.

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