CHAR(19)太小，无法以毫秒为单位存储整个日期.

CHAR(19) is too small to store your whole date with milliseconds.

改为使用CHAR(26):

Use CHAR(26) instead:

CREATE TEMPORARY TABLE TMP(
    SELECT CAST(DATE_FORMAT(NOW(), '%Y-%m-%d %T.%f') AS CHAR(26)) AS REGDT
);

或删除毫秒:

CREATE TEMPORARY TABLE TMP(
    SELECT CAST(DATE_FORMAT(NOW(), '%Y-%m-%d %T') AS CHAR(19)) AS REGDT
);

您还可以使用DATETIME类型代替String.

You could also use a DATETIME type instead of a String.