一.Mybatis-Plus——sum聚合函数
//总收益
Order order =new Order();
QueryWrapper<Order> queryWrapper = new QueryWrapper<Order>();
queryWrapper.select("sum(price) as sumAll");
Order ord= orderService.getOne(queryWrapper);
//注意,空指针问题
if (ord== null){
order.setSumAll(Double.valueOf(0)) ;
}else{
order.setSumAll(ord.getSumAll());
}
对应的sql语句是
SELECT sum(price) as sumAll FROM t_order
注意:as后面的变量要与实体类中声明的一致, @TableField(exist = false)表示表中并没有相应的字段与之对应。
(queryWrapper的getOne()方法:根据 Wrapper,查询一条记录)
/**
* 总收益
*/
@TableField(exist = false)
private Double sumAll;
参考:https://blog.csdn.net/weixin_45616483/article/details/105540393?utm_medium=distribute.pc_relevant_t0.none-task-blog-BlogCommendFromMachineLearnPai2-1.compare&depth_1-utm_source=distribute.pc_relevant_t0.none-task-blog-BlogCommendFromMachineLearnPai2-1.compare
二.按日期查询并求和(mybatis及mybatisPlus两种方式)
举例: 今日总收益
1.mybatisPlus,只需在controller层进行代码书写
//今日总收益
QueryWrapper<Order> queryWrapper = new QueryWrapper<Order>();
queryWrapper.select("sum(price) as sumByToday ");
queryWrapper.eq("TO_DAYS(create_time)","TO_DAYS(NOW())");
Order order =new Order();
if (orderService.getOne(queryWrapper) == null){
order.setSumByToday(Double.valueOf(0)) ;
}else{
order.setSumByToday(orderService.getOne(queryWrapper).getSumByToday());
}
2.mybatis (自己定义sql,简单灵活,但代码量相较于mybatisPlus多了几行)
controller层
//今日总收益
Double sumByToday =orderService.sumByToday();
if (sumByToday == null){
sumByToday = Double.valueOf(0);
}
order.setSumByToday(sumByToday);
service层
Double sumByToday();
service实现层
@Override
public Double sumByToday(){
return orderMapper.sumByToday();
}
mapper层
Double sumByToday();
xml文件
<select id="sumByToday" resultType="java.lang.Double">
select sum(price) as sumByToday from t_order
where TO_DAYS(create_time) = TO_DAYS(NOW())
</select>
3.bug记录,错误信息如下:
Mapper method 'mapper.OrderMapper.sumByToday attempted to return null from a method with a primitive return type (double).
查询出来的值为空,它的数据为null而不是double数据。
解决方法:
方法1:实体类中用来接收“今日总收益“的变量,类型变成封装类Double(Integer、Long,而不是double。
方法2:xml文件中的sql语句变成
select IFNULL(sum(price),0) as sumByToday
from t_order
where TO_DAYS(create_time) = TO_DAYS(NOW())
IFNULL(expression, value):如果第一个参数的表达式 expression 为 NULL,则返回第二个参数的备用值。