左外部联接

可以看到和直接内连接的join差距在多了into，把可能为空的那个集合(表)放到一个集合，然后再对接进行DefaultIfEmpty()，再从这个结果中去取

stocksinfo表格

packetassociationstocks 表格

实现 stocksinfo表格和packetassociationstocks 表格的左外连接，并且选择 Join后的前10条记录

    /// <summary>
        ///  为指定的板块添加股票
        /// </summary>
        /// <param name="blockID">板块id</param>
        /// <param name="keyword">查找关键词</param>
        /// <returns></returns>
        public async Task<ObservableCollection<StockPOCO>?> GetStocksForPacketAsync(int blockID,string keyword)
        {
            ObservableCollection<StockPOCO>? coll = new();
            //选取股票名称或者名字 保护指定关键词的 选取前十条记录
            var content = (from A in dataContext.Stocksinfos
                           join B in dataContext.Packetassociationstocks on A.Stockid equals B.Stockid into prodGroupList
                           from C in prodGroupList.DefaultIfEmpty()//DefaultIfEmpty()实现左外连接
                           where A.Stockid.Contains(keyword) || A.Name.Contains(keyword)// 筛选 表格A，切记不是筛选prodGroupList
                           select new StockPOCO{ StockID= A.Stockid,  Name= A.Name, HadAdded= C.Isselected==1 }
                          ).Take(10).ToList();
            return new ObservableCollection<StockPOCO>(content);
        }