查阅官档：

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Returns a representation of the unix_timestamp argument as a value in 'YYYY-MM-DD HH:MM:SS' orYYYYMMDDHHMMSS format, depending on whether the function is used in a string or numeric context. The value is expressed in the current time zone. unix_timestamp is an internal timestamp value such as is produced by the function.

说明from_unixtime只是按照当前的时区，已指定的形式返回unix_timestamp值。

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If called with no argument, returns a Unix timestamp (seconds since '1970-01-01 00:00:00' UTC) as an unsigned integer. If  is called with a date argument, it returns the value of the argument as seconds since '1970-01-01 00:00:00' UTC. date may be a  string, a  string, a , or a number in the format YYMMDD or YYYYMMDD. The server interprets date as a value in the current time zone and converts it to an internal value in UTC

不同点在与，unix_timestamp会受到本地时区的影响，有可能2个不同的timstamp对应一个unixtimestamp，而from_unixtime则是一一对应