lambda 表达式，它可以让我们避免许多重复性的工作。Java中的 lambda 表达式实质上是一个匿名方法，但该方法并非独立执行，而是用于实现由函数式接口定义的唯一抽象方法。

Mybatis-plus中的lambda主要有QueryWrapper(LambdaQueryWrapper) 和 UpdateWrapper(LambdaUpdateWrapper) 的父类用于生成 sql 的 where 条件, entity 属性也用于生成 sql 的 where 条件注意: entity 生成的 where 条件与 使用各个 api 生成的 where 条件没有任何关联行为

LambdaQueryWrapper

lambda 构建复杂的查询条件构造器：LambdaQueryWrapper，下面是常见的构造方式

LambdaQueryWrapper<UserEntity> lambda = new QueryWrapper<UserEntity>().lambda();

LambdaQueryWrapper<UserEntity> lambda = new  LambdaQueryWrapper<>();

LambdaQueryWrapper<UserEntity> lambda = Wrappers.lambdaQuery();

List<UserEntity> users = new LambdaQueryChainWrapper<UserEntity>(userMapper)
            .like(User::getName, "雨").ge(User::getAge, 20).list();

LambdaUpdateWrapper

LambdaUpdateWrapper<UserEntity> luw = Wrappers.lambdaUpdate();

@Test
public void testLambdaUpdate() {
    LambdaUpdateWrapper<UserEntity> luw = Wrappers.lambdaUpdate();
    luw.set(UserEntity::getUserName, "dunzung01")
            .set(UserEntity::getSex, 1);
    luw.eq(UserEntity::getUserId, 1);
    userMapper.update(null, luw);
}

常用的条件构造器

setSqlSelect	设置 SELECT 查询字段
where	WHERE 语句，拼接 + WHERE 条件
and	AND 语句，拼接 + AND 字段=值
andNew	AND 语句，拼接 + AND (字段=值)
or	OR 语句，拼接 + OR 字段=值
orNew	OR 语句，拼接 + OR (字段=值)
eq	等于=
allEq	基于 map 内容等于=
ne	不等于<>
gt	大于>
ge	大于等于>=
lt	小于<
le	小于等于<=
like	模糊查询 LIKE
notLike	模糊查询 NOT LIKE
likeLeft 左通配符模糊查询
likeRight 右通配符模糊查询    
in	IN 查询
notIn	NOT IN 查询
isNull	NULL 值查询
isNotNull	IS NOT NULL
groupBy	分组 GROUP BY
having	HAVING 关键词
orderBy	排序 ORDER BY
orderAsc	ASC 排序 ORDER BY
orderDesc	DESC 排序 ORDER BY
exists	EXISTS 条件语句
notExists	NOT EXISTS 条件语句
between	BETWEEN 条件语句
notBetween	NOT BETWEEN 条件语句
addFilter	自由拼接 SQL
last	拼接在最后，例如：last(“LIMIT 1”)

常用实例

//等值查询：eq
@Test
public void testLambdaQueryOfEq() {
    //eq查询
    //至关于 select * from sys_user where user_id = 1
    LambdaQueryWrapper<UserEntity> lqw = Wrappers.lambdaQuery();
    lqw.eq(UserEntity::getUserId, 1L);
    UserEntity user = userMapper.selectOne(lqw);
    System.out.println("eq查询::" + user.getUserName());
}

//范围查询 ：in
@Test
public void testLambdaQueryOfIn() {  
    List<Long> ids = Arrays.asList(1L, 2L);
    LambdaQueryWrapper<UserEntity> lqw = Wrappers.lambdaQuery();  
    lqw.in(UserEntity::getUserId, ids);
    List<UserEntity> userList = userMapper.selectList(lqw);
    userList.forEach(u -> System.out.println("in查询::" + u.getUserName()));
}

//通配符模糊查询：like
@Test
public void testLambdaQueryOfLikeAll() {
    LambdaQueryWrapper<UserEntity> lqw = Wrappers.lambdaQuery();
    lqw.eq(UserEntity::getSex, 0L)
            .like(UserEntity::getUserName, "dun");
    List<UserEntity> userList = userMapper.selectList(lqw);
    userList.forEach(u -> System.out.println("like全包含关键字查询::" + u.getUserName()));
}

//右通配符模糊查询：likeRight
@Test
public void testLambdaQueryOfLikeRight() { 
    LambdaQueryWrapper<UserEntity> lqw = Wrappers.lambdaQuery();
    lqw.eq(UserEntity::getSex, 0L)
            .likeRight(UserEntity::getUserName, "dun");
    List<UserEntity> userList = userMapper.selectList(lqw);
    userList.forEach(u -> System.out.println("like Right含关键字查询::" + u.getUserName()));
}

//左通配符模糊查询：likeLeft
@Test
public void testLambdaQueryOfLikeLeft() {  
    LambdaQueryWrapper<UserEntity> lqw = Wrappers.lambdaQuery();
    lqw.eq(UserEntity::getSex, 0L)
            .likeLeft(UserEntity::getUserName, "zung");
    List<UserEntity> userList = userMapper.selectList(lqw);
    userList.forEach(u -> System.out.println("like Left含关键字查询::" + u.getUserName()));
}


//利用 or 和 and 构建复杂的查询条件
@Test
public void testLambdaQueryOfOr_And() {  
    LambdaQueryWrapper<UserEntity> lqw = Wrappers.lambdaQuery();
    lqw.eq(UserEntity::getSex, 0L)
            .and(wrapper->wrapper.eq(UserEntity::getUserName,"dunzung")
                    .or().ge(UserEntity::getAge, 50));
    List<UserEntity> userList = userMapper.selectList(lqw);
    userList.forEach(u -> System.out.println("like查询::" + u.getUserName()));
}

//善于利用分页利器 PageHelpler
@Test
public void testLambdaPage() {
    //PageHelper分页查询
    //至关于 select * from sys_user limit 0,2
    int pageNumber = 0;
    int pageSize = 2;
    PageHelper.startPage(pageNumber + 1, pageSize);
    LambdaQueryWrapper<UserEntity> lqw = Wrappers.lambdaQuery();
    lqw.orderByAsc(UserEntity::getAge)
          .orderByDesc(UserEntity::getMobile);
    List<UserEntity> userList = userMapper.selectList(lqw); 
    userList.forEach(u -> System.out.println("page分页查询::" + u.getUserName()));
}