mysql没没有声明主键会自动选择一列不重复的作为聚集索引，怎么找到它默认生成的是哪一列？

Question

mysql没没有声明主键会自动选择一列不重复的作为聚集索引，怎么找到它默认生成的是哪一列？

Answer 1

If you do not define a PRIMARY KEY for your table, MySQL picks the first UNIQUE index that has only NOT NULL columns as the primary key and InnoDB uses it as the clustered index. If there is no such index in the table, InnoDB internally generates a clustered index where the rows are ordered by the row ID that InnoDB assigns to the rows in such a table. The row ID is a 6-byte field that increases monotonically as new rows are inserted. Thus, the rows ordered by the row ID are physically in insertion order.