Syntax:
CONVERT(expr USING transcoding_name), CONVERT(expr,type)

The CONVERT() function takes an expression of any type and produces a
result value of the specified type.

CONVERT(... USING ...) is standard SQL syntax. The non-USING form of
CONVERT() is ODBC syntax.

CONVERT(expr USING transcoding_name) converts data between different
character sets. In MySQL, transcoding names are the same as the
corresponding character set names. For example, this statement converts
the string 'abc' in the default character set to the corresponding
string in the utf8 character set:

SELECT CONVERT('abc' USING utf8);

CONVERT(expr, type) syntax (without USING) takes an expression and a
type value specifying the result type. This operation may also be
expressed as CAST(expr AS type), which is equivalent. These type values
are permitted:

o BINARY[(N)]

  Produces a string with the BINARY data type. For a description of how
  this affects comparisons, see
  https://dev.mysql.com/doc/refman/5.7/en/binary-varbinary.html. If the
  optional length N is given, BINARY(N) causes the cast to use no more
  than N bytes of the argument. Values shorter than N bytes are padded
  with 0x00 bytes to a length of N.

o CHAR[(N)] [charset_info]

  Produces a string with the CHAR data type. If the optional length N
  is given, CHAR(N) causes the cast to use no more than N characters of
  the argument. No padding occurs for values shorter than N characters.

  With no charset_info clause, CHAR produces a string with the default
  character set. To specify the character set explicitly, these
  charset_info values are permitted:

  o CHARACTER SET charset_name: Produces a string with the given
    character set.

  o ASCII: Shorthand for CHARACTER SET latin1.

  o UNICODE: Shorthand for CHARACTER SET ucs2.

  In all cases, the string has the character set default collation.

o DATE

  Produces a DATE value.

o DATETIME

  Produces a DATETIME value.

o DECIMAL[(M[,D])]

  Produces a DECIMAL value. If the optional M and D values are given,
  they specify the maximum number of digits (the precision) and the
  number of digits following the decimal point (the scale).

o JSON

  Produces a JSON value. For details on the rules for conversion of
  values between JSON and other types, see
  https://dev.mysql.com/doc/refman/5.7/en/json.html#json-comparison.

o NCHAR[(N)]

  Like CHAR, but produces a string with the national character set. See
  https://dev.mysql.com/doc/refman/5.7/en/charset-national.html.

  Unlike CHAR, NCHAR does not permit trailing character set information
  to be specified.

o SIGNED [INTEGER]

  Produces a signed integer value.

o TIME

  Produces a TIME value.

o UNSIGNED [INTEGER]

  Produces an unsigned integer value.

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

