MySql如何将varchar（纬度，经度）转换为十进制字段？

In mysql I have a varchar containing latitude and longitude provided by Google maps. I need to be able to query based on a bounding box value, but have no need for the geo features now available. I'm trying to populate 2 new Decimal fields with the Decimal values found in the varchar. Here is the query that I'm trying to use, but the result are all rounded values in the new fields.In mysql I have a varchar containing latitude a