MP其它使用条件构造器的方法——selectMaps、selectObjs、selectCount、selectOne

一、selectMaps方法

应用场景一：表字段特别多，但是需求只要其中几个时，使用selectMaps更优雅

//    SELECT id,name FROM user WHERE (name LIKE ? AND age < ?) 
    @Test
    public void selectByWrapperMaps(){
        QueryWrapper<User> queryWrapper = new QueryWrapper<>();

        queryWrapper.select("id","name").like("name","J").lt("age",40);

        List<Map<String, Object>> userList = userMapper.selectMaps(queryWrapper);
        userList.forEach(System.out::println);
    }

应用场景二：统计查询中返回统计结果时

//    SELECT avg(age) avg_age,min(age) min_age,max(age) max_age FROM user GROUP BY age HAVING sum(age) > ?
    @Test
    public void selectByWrapperMaps2(){
        QueryWrapper<User> queryWrapper = new QueryWrapper<>();

        queryWrapper.select("avg(age) avg_age","min(age) min_age","max(age) max_age")
                .groupBy("age").having("sum(age) > {0}", 20);

        List<Map<String, Object>> userList = userMapper.selectMaps(queryWrapper);
        userList.forEach(System.out::println);
    }

二、selectObjs方法

特点：只返回第一个字段的值，注意不是行，是列；比如上面例子就只返回id列的几个值；泛型因为不确定第一列会是什么类型，所以也是用Object类型。

List<Object> userList = userMapper.selectObjs(queryWrapper);

三、selectCount方法

特点：会自动生成select（count（1）），不能使用自定义select字段了，用的话会报错。

//    SELECT COUNT( 1 ) FROM user WHERE (name LIKE ? AND age < ?) 
    @Test
    public void selectByWrapperCount(){
        QueryWrapper<User> queryWrapper = new QueryWrapper<>();

        queryWrapper.like("name","J").lt("age",40);

        Integer userList = userMapper.selectCount(queryWrapper);
        System.out.println("总记录数为："+userList);
    }

四、selectOne方法

特点：只返回一条结果，查询结果必须是一条或者没有，如果有多条结果，会报错

//    SELECT id,name,age,email FROM user WHERE (name LIKE ? AND age < ?) 
    @Test
    public void selectByWrapperOne(){
        QueryWrapper<User> queryWrapper = new QueryWrapper<>();

        queryWrapper.like("name","Jack").lt("age",40);

        User userList = userMapper.selectOne(queryWrapper);
        System.out.println(userList);
    }