mysql默认的排序：
https://forums.mysql.com/read.php?21,239471,239688#msg-239688
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Do not depend on order when ORDER BY is missing.
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Always specify ORDER BY if you want a particular order -- in some situations the engine can eliminate the ORDER BY because of how it does some other step.
- GROUP BY forces ORDER BY. (This is a violation of the standard. It can be avoided by using ORDER BY NULL.)
SELECT * FROM tbl -- this will do a "table scan". If the table has never had any DELETEs/REPLACEs/UPDATEs, the records will happen to be in the insertion order, hence what you observed.
If you had done the same statement with an InnoDB table, they would have been delivered in PRIMARY KEY order, not INSERT order. Again, this is an artifact of the underlying implementation, not something to depend on.
https://stackoverflow.com/questions/8746519/sql-what-is-the-default-order-by-of-queries