Comments - User-Defined Variables
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Beware order of evaluation of rows when sorting. It would appear (at least as far as I've tested it) that when statements of the form:
SELECT @var:=<expression> FROM...
are executed, @var will always correspond to the value in the last row returned, as you might expect.
But if you do something like:
SELECT IFNULL(@var:=Name,'unknown') FROM Customers ORDER BY 1 LIMIT 20
The statement containing the variable assignment will be executed *before* the order-by. In this case, the value left in @var at the end might not even correspond to any of the rows returned!
It would be good if the MariaDB docs could clarify under exactly what conditions the order-of-variable-assignments corresponds to the order-of-returned rows, and especially under what conditions this differs!