How to use having on un-aggregate field in MySQL?

select xx as fieldA from ... group by xxx having fieldA is not null

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I found that having has no effect

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May be :

 select xx as fieldA from ... where fieldA is not null group by xxx having 

help you

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You should include all non-aggregate columns in the GROUP BY. 5.7.5 introduced the SQL_MODE of only_full_group_by to enforce such (while not complaining about some cases where it is actually OK).

When you have a non-aggregate column, the code is free to provide any value it likes for such a column (xx in your case). Given that, then how could the HAVING make any rational sense.

This is not a bug; it is a user error.