MBP02 Basic select queries in Mybatis-plus

Mapper中的基本查询

Continue to use the database table and entity class created in the last blog:

create table users (
  id integer primary key not null auto_increment,
  username varchar(50) not null,
  password varchar(100) not null,
  question varchar(150),
  answer varchar(50)
);

@TableName(value = "users")
public class AppUser {
    @TableId(value = "id", type = IdType.AUTO)
    private Integer id;
    private String username;
    private String password;
    private String question;
    private String answer;
}

public interface AppUserMapper extends BaseMapper<AppUser> {
}

selectById 与 selectBatchIds

AppUser appUser = appUserMapper.selectById(2);

List<Integer> ids = new ArrayList<>();
ids.add(1);
ids.add(2);
ids.add(3);
List<AppUser> users = appUserMapper.selectBatchIds(ids);

通过Map - selectByMap

通过Map来封装查询条件，map中写的是数据表中的列名，而非实体类的属性名。

Map<String,Object> columnMap = new HashMap<>();
columnMap.put("username", "jisu");
columnMap.put("question", "What is my name?");
List<UserInfo> users = userInfoMapper.selectByMap(columnMap);

使用QueryWrapper

QueryWrapper是一个查询构造器，相当于Where条件。

QueryWrapper<AppUser> queryWrapper = new QueryWrapper<>();
queryWrapper.like("username","ha").like("question", "What");

QueryWrapper的方法见下表：

method name	note	example
eq	=	eq("name", "wong")
ne	!=	ne("name", "wong")
gt	>	gt("age", 18)
ge	>=
lt	<
le	<=
or	OR	紧接着的下一个方法不是用ADD连接eq("id", 1).or().eq(age, 18)
between
notBetween
like
notLike
likeLeft
likeRight
inSql
notInSql
groupBy
orderByAsc
orderByDesc