SELECT km.menusCateId, km.menusCateName, km.parentId, km.sort, km.menusId, km.mUrl mUrl, km.mIco mIco, km.lcCode, km.pubId, km.updId, km.creDate, km.updDate --select COUNT(0) over partition by km.menusCateName FROM ka_menusCate km LEFT JOIN ka_lifeMenu kl ON km.menusId = kl.menusId

这是一个 SQL 查询语句，它的功能是从 ka_menusCate 和 ka_lifeMenu 两个表中查询数据。该查询语句使用了 LEFT JOIN 连接两个表，并选择了一些列作为输出结果，包括 menusCateId, menusCateName, parentId, sort, menusId, mUrl, mIco, lcCode, pubId, updId, creDate 和 updDate。同时，该查询语句在 ka_menusCate 表中按照 menusCateName 列进行分组，并统计每组中的记录数。

相关问题

public class VMenuInfo : MenuInfo { public MenuInfo _menuInfo { get; set; } public VMenuInfo() { //_menuInfo = this; base.Mkey = _menuInfo.Mkey; base.MUrl = _menuInfo.MUrl; base.Create = _menuInfo.Create; base.MOrder = _menuInfo.MOrder; base.iconUrl = _menuInfo.iconUrl; base.ChangeId = _menuInfo.ChangeId; base.ChangeTime = _menuInfo.ChangeTime; base.IsDeleted = _menuInfo.IsDeleted; base.MId = _menuInfo.MId; base.ParentId = _menuInfo.ParentId; base.MName = _menuInfo.MName; base.ParentName = _menuInfo.ParentName; base.IsTop = _menuInfo.IsTop; base.CreateTime = _menuInfo.CreateTime; } public List<VMenuInfo> children { get; set; } }改进这段代码

首先，无论是继承类还是组合类，都需要在构造函数中对基类或者组合对象进行初始化。但是，在这个代码中，由于没有对_menuInfo进行初始化，所以会导致_nullReferenceException_异常，因此需要在构造函数中给_menuInfo对象赋值：

public class VMenuInfo : MenuInfo
{
public MenuInfo _menuInfo { get; set; }

public VMenuInfo()
{
_menuInfo = new MenuInfo(); //初始化_menuInfo对象
base.Mkey = _menuInfo.Mkey;
base.MUrl = _menuInfo.MUrl;
base.Create = _menuInfo.Create;
base.MOrder = _menuInfo.MOrder;
base.iconUrl = _menuInfo.iconUrl;
base.ChangeId = _menuInfo.ChangeId;
base.ChangeTime = _menuInfo.ChangeTime;
base.IsDeleted = _menuInfo.IsDeleted;
base.MId = _menuInfo.MId;
base.ParentId = _menuInfo.ParentId;
base.MName = _menuInfo.MName;
base.ParentName = _menuInfo.ParentName;
base.IsTop = _menuInfo.IsTop;
base.CreateTime = _menuInfo.CreateTime;
}

public List<VMenuInfo> children { get; set; }
}

另外，由于VMenuInfo及其子类会使用到MenuInfo中的所有属性，因此可以考虑将VMenuInfo中的属性和MenuInfo保持一致，避免不必要的数据冗余和查询更新操作。最终的代码如下：

public class MenuInfo
{
public string Mkey { get; set; }
public string MUrl { get; set; }
public string Create { get; set; }
public int MOrder { get; set; }
public string iconUrl { get; set; }
public long ChangeId { get; set; }
public DateTime ChangeTime { get; set; }
public bool IsDeleted { get; set; }
public int MId { get; set; }
public int ParentId { get; set; }
public string MName { get; set; }
public string ParentName { get; set; }
public bool IsTop { get; set; }
public DateTime CreateTime { get; set; }
}

public class VMenuInfo : MenuInfo
{
public List<VMenuInfo> Children { get; set; }
}

解释这段语句 with recursive t1 as ( select * from xc402_content.course_category where id ='1'union all select t2.* from xc402_content.course_category t2 inner join t1 on t1.id=t2.parentid ) select * from t1 order by t1.id

这是一段使用递归查询的 SQL 语句。它的目的是从名为 `xc402_content.course_category` 的表中获取 id 为 '1' 的课程类别及其所有子类别，并按照 id 排序进行返回。

具体地，这个查询语句定义了一个递归的临时表 `t1`，它首先选择了 `xc402_content.course_category` 表中 id 为 '1' 的记录。然后，通过内连接将这些记录与 `t1` 表进行连接，条件是 `t1` 的 id 等于 `t2` 的 parentid。这样，每次递归都会在结果集中添加更多满足条件的记录，直到没有更多记录符合条件为止。

最后，通过在 `t1` 表上执行 `select *` 查询，并按照 id 进行排序，返回最终结果。