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List<SysUser> list=QueryChain.of(sysUserMapper)
    .select(SysUser.class)
    .from(SysUser.class)
    .join(SysUser.class,SysRole.class)
    .list();

List<SysUser> list=QueryChain.of(sysUserMapper)
    .select(SysUser.class)
    .from(SysUser.class)
    .join(SysUser.class,1,SysUser.class,2,on->on.eq(SysUser::getId,1,SysUser::getRole_id,2))
    .list();

SubQuery subQuery=SubQuery.create("sub")
    .select(SysRole.class)
    .from(SysRole.class)
    .eq(SysRole::getId,1);

List<SysUser> list=QueryChain.of(sysUserMapper)
    .select(SysUser.class)
    .from(SysUser.class)
    .join(SysUser.class,subQuery,on->on.eq(SysUser::getRole_id,subQuery.$(subQuery,SysRole::getId)))
    .list();

返回2张相同表字段​

List<SysUserVO> list=QueryChain.of(sysUserMapper)
    .select(SysUser.class)
    .select(SysUser.class,2)   
    .from(SysUser.class)
    .join(SysUser.class,1,SysUser.class,2,on->on.eq(SysUser::getId,1,SysUser::getRole_id,2))
    .returnType(SysUserVO.class)    
    .list();

ORM join 好不好用 就看 你用它 join 自己的处理 好不好 一看便知！！！