先看一个示例：
数据表数据：
mybatis类和查询语句：
1. 当UserInfoMap中所有字段(包含association)都为NULL的话，getUserInfo的返回结果是个null，即使查询的记录存在！运行结果如下：
2019-06-26 10:50:00.800 DEBUG 95032 --- [ main] com.zcm.mybatis.dao.UserDao.getUserInfo : ==> Preparing: select name,phone,info_id from d_user where id = ?
2019-06-26 10:50:00.854 DEBUG 95032 --- [ main] com.zcm.mybatis.dao.UserDao.getUserInfo : ==> Parameters: 1(Integer)
2019-06-26 10:50:00.954 DEBUG 95032 --- [ main] c.z.m.d.m.InfoMapper.selectByPrimaryKey : ====> Preparing: select id, card_id, birth, weight, height from d_info where id = ?
2019-06-26 10:50:00.955 DEBUG 95032 --- [ main] c.z.m.d.m.InfoMapper.selectByPrimaryKey : ====> Parameters: 0(Integer)
2019-06-26 10:50:00.956 DEBUG 95032 --- [ main] c.z.m.d.m.InfoMapper.selectByPrimaryKey : <==== Total: 0
2019-06-26 10:50:00.957 DEBUG 95032 --- [ main] com.zcm.mybatis.dao.UserDao.getUserInfo : <== Total: 1
null2. 当UserInfoMap中有字段不为NULL的时候，getUserInfo返回的结果才不为null(前提查询的记录存在)，如d_user表id=1中，为phone添加值"x",如下图：
这时再运行测试程序，结果如下：
2019-06-26 11:01:20.794 DEBUG 95861 --- [ main] com.zcm.mybatis.dao.UserDao.getUserInfo : ==> Preparing: select name,phone,info_id from d_user where id = ?
2019-06-26 11:01:20.856 DEBUG 95861 --- [ main] com.zcm.mybatis.dao.UserDao.getUserInfo : ==> Parameters: 1(Integer)
2019-06-26 11:01:20.965 DEBUG 95861 --- [ main] c.z.m.d.m.InfoMapper.selectByPrimaryKey : ====> Preparing: select id, card_id, birth, weight, height from d_info where id = ?
2019-06-26 11:01:20.966 DEBUG 95861 --- [ main] c.z.m.d.m.InfoMapper.selectByPrimaryKey : ====> Parameters: 0(Integer)
2019-06-26 11:01:20.967 DEBUG 95861 --- [ main] c.z.m.d.m.InfoMapper.selectByPrimaryKey : <==== Total: 0
2019-06-26 11:01:20.968 DEBUG 95861 --- [ main] com.zcm.mybatis.dao.UserDao.getUserInfo : <== Total: 1
{"phone":"x"}所以综合本测试案例，得出的最终结果是：
1. 记录存在时，不一定返回的对象一定不为null, 还要看查询的字段是否全部为NULL，或者说是否有不为NULL的字段！
2. 这个测试结果跟是否有关联查询无关!
使用简介)
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