用于检查表中一列数据的 C# 代码是否存在于其他表中答案

【问题标题】：C# code to check one columns data from table has exist in other tables用于检查表中一列数据的 C# 代码是否存在于其他表中
【发布时间】：2020-06-28 11:53:36
【问题描述】：

我想检查表A 中的一条记录以检查它是否存在于表B 中。我计算表B中输入了多少数据。

例子：

   A                       B
------------------------------
John                     John
Joon                     Jorge
Jorge                    Joon
Elizabet                 Elizabet
Suzan                    Suzan
                         Elizabet
                         Joon
                         Suzan
                         John
                         Elizabet

答案应该是这样的：

John = 2
Joon = 2
jorge = 1
Elizabet = 3
Suzan = 2

我尝试过的：

using (ImportInvoiceMasterForm.ApplicationSqlConnection2 = new SqlConnection(MainForm.ApplicationDataBase))
            {
                ImportInvoiceMasterForm.ApplicationSqlConnection2.Open();
                using (ImportInvoiceMasterForm.ApplicationSqlCommand1 = new SqlCommand("SELECT * FROM TBL_Stock_Item", ImportInvoiceMasterForm.ApplicationSqlConnection2))
                using (ImportInvoiceMasterForm.ApplicationSqlDataReader1 = ImportInvoiceMasterForm.ApplicationSqlCommand1.ExecuteReader())
                {
                    while (ImportInvoiceMasterForm.ApplicationSqlDataReader1.Read())
                    {
                        ImportInvoiceMasterForm.GetImportQuantity = 0;
                        #region Get Import Quantity
                        using (ImportInvoiceMasterForm.ApplicationSqlConnection1 = new SqlConnection(MainForm.ApplicationDataBase))
                        {
                            ImportInvoiceMasterForm.ApplicationSqlConnection1.Open();
                            using (ImportInvoiceMasterForm.ApplicationSqlCommand = new SqlCommand("SELECT * FROM TBL_Import_Items WHERE ImportItemName='" + ImportInvoiceMasterForm.ApplicationSqlDataReader1.GetString(1) + "'", ImportInvoiceMasterForm.ApplicationSqlConnection1))
                            using (ImportInvoiceMasterForm.ApplicationSqlDataReader = ImportInvoiceMasterForm.ApplicationSqlCommand.ExecuteReader())
                            {
                                while (ImportInvoiceMasterForm.ApplicationSqlDataReader.Read())
                                {
                                    GetImportQuantity += double.Parse(ImportInvoiceMasterForm.ApplicationSqlDataReader.GetValue(4).ToString());
                                }
                            }
                        }
                        #endregion Get Import Quantity
                    }
                }
            }

【问题讨论】：

你为什么不只在查询中这样做？我建议你使用存储过程并在那里编写逻辑

标签： c# sql

【解决方案1】：

这看起来像一个聚合。一种方法是join/group by:

select a.name, count(b.name)
from a left join
     b
     on a.name = b.name
group by a.name;

left join 允许0 的计数。

如果名称可以在a 中重复，那么关联子查询是更好的解决方案（假设您希望在a 中保留重复项）：

select a.*, (select count(*) from b where b.name = a.name) as cnt
from a;

【讨论】：

【解决方案2】：

Select A.name, Count(B.name) As Num
FROM A join B 
Where B.name=A.name
Group By A.name

这将为您提供所需的解决方案

【讨论】：