Syntax:
FORMAT(X,D[,locale])

Formats the number X to a format like '#,###,###.##', rounded to D
decimal places, and returns the result as a string. If D is 0, the
result has no decimal point or fractional part.

The optional third parameter enables a locale to be specified to be
used for the result number's decimal point, thousands separator, and
grouping between separators. Permissible locale values are the same as
the legal values for the lc_time_names system variable (see
https://dev.mysql.com/doc/refman/5.7/en/locale-support.html). If no
locale is specified, the default is 'en_US'.

URL: https://dev.mysql.com/doc/refman/5.7/en/string-functions.html

