MyBatis的Example类详解

古城微笑少年丶 2021-11-16 17:46 583阅读 0赞

关于Example类的定义？  
mybatis-generator会为每个字段产生Criterion，为底层的mapper.xml创建动态sql。如果表的字段比较多,产生的example类会十分庞大。理论上通过example类可以构造你想到的任何筛选条件。在mybatis-generator中加以配置，配置数据表的生成操作就可以自动生成example了。这里对定义类型的不再过多解释，直接上干货。  
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1.查询：  
(1)selectByPrimaryKey

User user = UserMapper.selectByPrimaryKey("1001");
    等同于：select * from user where id = "1001"

(2)selectByExample (and条件)

Example example = new Example(User.class);
    example.createCriteria().andEqualTo("id", "1001" )
    	.andEqualTo("userName", "小李");
    User user = UserMapper.selectByExample(example);
    
    等同于：select * from user where id = "1001" and username = '小李'

selectByExample (or条件)

Example example = new Example(User.class);
    example.or.andEqualTo("id", "1001" )
    example.or.andEqualTo("userName", "小李");
    User user = UserMapper.selectByExample(example);
    
    等同于：select * from user where id = "1001" or username = '小李'

selectByExample (and+or多条件查询)

Example example = new Example(User.class);
            example.createCriteria().andEqualTo("id", "1" )
                    .andEqualTo("userId", "1001");
            example.and().orEqualTo("userName","小李")
                    .orEqualTo("passWord","123");
            List<User> user = UserMapper.selectByExample(example);
    
    等同于：
    SELECT id,user_id,user_name,pass_word FROM user WHERE ( id = "1" and user_id = "1001" ) and ( user_name = "小李" or pass_word = "123" )

2.插入：

User user = new User();
    user.setId("2");
    user.setUserId("1002");
    user.setUserName("小王");
    user.setPassWord("666")
    UserMapper.insert(user);
    等同于：insert into user(id,user_id ,user_name,pass_word) values ('2','1002','小王','666');

3.更新：  
(1)updateByPrimaryKey

User user = new User();
    user.setId("2");
    user.setUserId("1002");
    user.setUserName("小王");
    user.setPassWord("666")
    UserMapper.updateByPrimaryKey(user);
    等同于：update user set user_id = "1002" user_name='小王', pass_word='666' where id='2'

(2)updateByExampleSelective

注意：updateByExample()更新所有的字段，包括字段为null的，建议使用 updateByExampleSelective()更新需要更新的字段
    
    Example example = new Example(User.class);
    example.createCriteria().andEqualTo("id","2");
    
    User user = new User();
    user.setUserName("小王");
    
    UserMapper.updateByExampleSelective(user,example);
    
    等同于：update user set user_name='小王' where id='2'

4.删除：  
(1)deleteByPrimaryKey

UserMapper.deleteByPrimaryKey("1");  //相当于：delete from user where id="1"

(2)deleteByExample

Example example = new Example(User.class);
    example.createCriteria().andEqualTo("userId","1002");
    UserMapper.deleteByExample(example);
    等同于：delete from user where userId='1002'

5.查询数量：

Example example = new Example(User.class);
    example.createCriteria().andEqualTo("id", "1001" )
    
    UserMapper.countByExample(example);
    等同于：select count(*) from user where id='1001'

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