SQL聚合函数中包含计算，列数据属性非数据格式则计算结果出错

在mysql中，同样在聚合函数中进行单位缩减，如果duration是varchar，计算会出错，duration是int就没问题

SELECT departure_city,SUM(duration) AS 'SUM' 
FROM `bus_route` 
GROUP BY departure_city;

SELECT departure_city,SUM(duration/1000) AS 'SUM' 
FROM `bus_route` 
GROUP BY departure_city;