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

1 SELECT departure_city,SUM(duration) AS 'SUM' 
2 FROM `bus_route` 
3 GROUP BY departure_city;
4 
5 SELECT departure_city,SUM(duration/1000) AS 'SUM' 
6 FROM `bus_route` 
7 GROUP BY departure_city;
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原文地址：https://www.cnblogs.com/wawawawa-briefnote/p/12112142.html