第1步：通过AutoWired注解实例化dao对象

//第1步：通过AutoWired注解实例化dao对象
@Autowired(required = false)
private UserDAO dao;

第2步: 创造条件构造器对象

QueryWrapper<User> queryWrapper = new QueryWrapper<>();

第3步: 使用构造器,构造sql语句

构造sql语句 select * from t_user where user_name= 'tom'

queryWrapper.eq("user_name","tom");

构造sql语句 select * from t_user where user_name= 'tom' and money between 700 and 1200

queryWrapper.between("money",700,1200);

第4步: 调用方法,执行构造的语句

//在执行第一个queryWrapper语句的基础上在执行第二个queryWrapper语句
List<User> list = dao.selectList(queryWrapper);
return list;

常见的sql语句构造方法

//通过AutoWired注解实例化dao对象
@Autowired(required = false)
private UserDAO dao;

//构造sql语句
@RequestMapping("/way2")
public List<User> fun2(){
    QueryWrapper<User> queryWrapper = new QueryWrapper<>();
    //查询姓名包含"张"
    queryWrapper.like("user_name","张");
    List<User> list = dao.selectList(queryWrapper);
    return list;
}

@RequestMapping("/way3")
public List<User> fun3(){
    QueryWrapper<User> queryWrapper = new QueryWrapper<>();
    //按money升序
    queryWrapper.orderByAsc("money");
    List<User> list = dao.selectList(queryWrapper);
    return list;
}

@RequestMapping("/way4")
public List<User> fun4(){
    QueryWrapper<User> queryWrapper = new QueryWrapper<>();
    //用 .or(). (或者)连接两个条件
    queryWrapper.eq("user_name","tom").or().between("money",700,1200);
    List<User> list = dao.selectList(queryWrapper);
    return list;
}

@RequestMapping("/way5")
public List<User> fun5(){
    QueryWrapper<User> queryWrapper = new QueryWrapper<>();
    //用 . (并且)连接两个条件
    queryWrapper.eq("user_name","tom").between("money",700,1200);
    List<User> list = dao.selectList(queryWrapper);
    return list;
}

@RequestMapping("/way6")
public List<User> fun6(){
    QueryWrapper<User> queryWrapper = new QueryWrapper<>();
    //子查询 inSql()
    //select * from t_user where id in (select id from t_user where money between 600 and 1000)
    queryWrapper.inSql("id","select id from t_user where money between 600 and 1000");
    List<User> list = dao.selectList(queryWrapper);
    return list;
}