sql - Error related to only_full_group_by when executing a query in MySql

I will try to explain you what this error is about.
Starting from MySQL 5.7.5, option ONLY_FULL_GROUP_BY is enabled by default.
Thus, according to standart SQL92 and earlier:

does not permit queries for which the select list, HAVING condition,
or ORDER BY list refer to nonaggregated columns that are neither named
in the GROUP BY clause nor are functionally dependent on (uniquely
determined by) GROUP BY columns

(read more in docs)

So, for example:

SELECT * FROM `users` GROUP BY `name`;

You will get error message after executing query above.

#1055 - Expression #1 of SELECT list is not in GROUP BY clause and contains nonaggregated column 'testsite.user.id' which is not
functionally dependent on columns in GROUP BY clause; this is
incompatible with sql_mode=only_full_group_by

Why?
Because MySQL dont exactly understand, what certain values from grouped records to retrieve, and this is the point.

I.E. lets say you have this records in your users table:

And you will execute invalid query showen above.
And you will get error shown above, because, there is 3 records with name John, and it is nice, but, all of them have different email field values.
So, MySQL simply don't understand which of them to return in resulting grouped record.

You can fix this issue, by simply changing your query like this:

SELECT `name` FROM `users` GROUP BY `name`

Also, you may want to add more fields to SELECT section, but you cant do that, if they are not aggregated, but there is crutch you could use (but highly not reccomended):

SELECT ANY_VALUE(`id`), ANY_VALUE(`email`), `name` FROM `users` GROUP BY `name`

Now, you may ask, why using ANY_VALUE is highly not recommended?
Because MySQL don't exactly know what value of grouped records to retrieve, and by using this function, you asking it to fetch any of them (in this case, email of first record with name = John was fetched).
Exactly I cant come up with any ideas on why you would want this behaviour to exist.

Please, if you dont understand me, read more about how grouping in MySQL works, it is very simple.

And by the end, here is one more simple, yet valid query.
If you want to query total users count according to available ages, you may want to write down this query

SELECT `age`, COUNT(`age`) FROM `users` GROUP BY `age`;

Which is fully valid, according to MySQL rules.
And so on.

It is important to understand what exactly the problem is and only then write down the solution.