Syntax:
FROM_UNIXTIME(unix_timestamp[,format])

Returns a representation of unix_timestamp as a datetime or character
string value. The value returned is expressed using the session time
zone. (Clients can set the session time zone as described in
https://dev.mysql.com/doc/refman/5.7/en/time-zone-support.html.)
unix_timestamp is an internal timestamp value representing seconds
since '1970-01-01 00:00:00' UTC, such as produced by the
UNIX_TIMESTAMP() function.

If format is omitted, this function returns a DATETIME value.

If unix_timestamp is an integer, the fractional seconds precision of
the DATETIME is zero. When unix_timestamp is a decimal value, the
fractional seconds precision of the DATETIME is the same as the
precision of the decimal value, up to a maximum of 6. When
unix_timestamp is a floating point number, the fractional seconds
precision of the datetime is 6.

format is used to format the result in the same way as the format
string used for the DATE_FORMAT() function. If format is supplied, the
value returned is a VARCHAR.

URL: https://dev.mysql.com/doc/refman/5.7/en/date-and-time-functions.html

