条件查询

1. QueryWrapper查询

@Test  
void Query(){  
    QueryWrapper qw = new QueryWrapper();  
    qw.lt("id",3);  
    List<User> l = userDao.selectList(qw);  
    System.out.println(l);  
}

2. Lambda

@Test  
void Query(){  
    QueryWrapper<User> qw = new QueryWrapper<User>();  
    qw.lambda().lt(User::getId,2);  
    List<User> l = userDao.selectList(qw);  
    System.out.println(l);  
}

这种方式是为了避免手动填写列名出错，如果你有十足把握不出错，也能这样写。

3. LambdaQueryWrapper

@Test  
void Query(){  
    LambdaQueryWrapper<User> lqw = new LambdaQueryWrapper<User>();  
    lqw.lt(User::getId,2);  
    List<User> l = userDao.selectList(lqw);  
    System.out.println(l);  
}

这种方式比较常用，省略了第二种方式的步骤

AND多条件查询：支持链式调用如 lqw.lt(User::getId,2).gt(User::getId,0) OR: lqw.lt(User::getId,2).or().gt(User::getId,0)

Null条件判断

LambdaQueryWrapper

@Test  
void Query(){  
    LambdaQueryWrapper<User> lqw = new LambdaQueryWrapper<User>();  
    lqw.lt(User::getId,2);  
    List<User> l = userDao.selectList(lqw);  
    System.out.println(l);  
}

Count(*)

@Test  
void Query(){  
    QueryWrapper<User> lqw = new QueryWrapper<User>();  
    lqw.select("count(*) as count, username");  
    lqw.groupBy("username");  
    List<Map<String,Object>> l = userDao.selectMaps(lqw);  
    System.out.println(l);  
}