mysql 小数点，直接截取小数位
SELECT CAST(SUBSTRING(CAST(3.141592699 AS CHAR(40)),1,LOCATE('.',CAST(3.141592699 AS CHAR(40)))+6) AS DECIMAL(20,6));
SELECT CAST(SUBSTRING(CAST(3.141592699 AS CHAR(40)),1,LOCATE('.',CAST(3.141592699 AS CHAR(40)))+6) AS DECIMAL(20,6));