ExecuteScalar()无结果集时返回DBNull.value

if (sqlcmd.ExecuteScalar() != DBNull.Value)
 {

     id = (int)sqlcmd.ExecuteScalar();
 }
      else

{
     id = 0;

}

如果找不到结果集中第一行的第一列;则返回 null 引用(在 Visual Basic 中为 Nothing)。 如果数据库中的该值为nullNothingnullptrnull 引用(在 Visual Basic 中为 Nothing) ,此查询将返回 DBNull.Value

 

详细请查看:IDbCommand.ExecuteScalar 方法

using System; using System.Collections.Generic; using System.Text; using System.Data; using System.Data.SqlClient; using System.Collections; using System.Configuration; namespace DBUtility { public class DbHelperSQL { // Fields public static string connectionString; // Methods static DbHelperSQL() { connectionString = ConfigurationManager.AppSettings["ConnectionString"]; } public DbHelperSQL() { } private static SqlCommand BuildIntCommand(SqlConnection connection, string storedProcName, IDataParameter[] parameters) { SqlCommand command = BuildQueryCommand(connection, storedProcName, parameters); command.Parameters.Add(new SqlParameter("ReturnValue", SqlDbType.Int, 4, ParameterDirection.ReturnValue, false, 0, 0, string.Empty, DataRowVersion.Default, null)); return command; } private static SqlCommand BuildQueryCommand(SqlConnection connection, string storedProcName, IDataParameter[] parameters) { SqlCommand command = new SqlCommand(storedProcName, connection); command.CommandType = CommandType.StoredProcedure; foreach (SqlParameter parameter in parameters) { if (parameter != null) { if (((parameter.Direction == ParameterDirection.InputOutput) || (parameter.Direction == ParameterDirection.Input)) && (parameter.Value == null)) { parameter.Value = DBNull.Value; } command.Parameters.Add(parameter); } } return command; } public static SqlDataReader ExecuteReader(string strSQL) { SqlDataReader dr; SqlConnection connection = new SqlConnection(connectionString); SqlCommand cmd = new SqlCommand(strSQL, connection); try { connection.Open(); dr= cmd.ExecuteReader(); } catch (SqlException e) { throw new Exception(e.Message); } return dr; } public static SqlDataReader ExecuteReader(string SQLString, params SqlParameter[] cmdParms) { SqlDataReader dr; SqlConnection connection = new SqlConnection(connectionString); SqlCommand cmd = new SqlCommand(); try { PrepareCommand(cmd, connection, null, SQLString, cmdParms); SqlDataReader myReader = cmd.ExecuteReader(); cmd.Parameters.Clear(); dr = myReader; } catch (SqlException e) { throw new Exception(e.Message); } return dr; } public static int ExecuteSql(string SQLString) { int count; using (SqlConnection connection = new SqlConnection(connectionString)) { using (SqlCommand cmd = new SqlCommand(SQLString, connection)) { try { connection.Open(); return cmd.ExecuteNonQuery(); } catch (SqlException E) { connection.Close(); throw new Exception(E.Message); } } } return count; } public static int ExecuteSql(string SQLString, string content) { int count; using (SqlConnection connection = new SqlConnection(connectionString)) { SqlCommand cmd = new SqlCommand(SQLString, connection); SqlParameter myParameter = new SqlParameter("@content", SqlDbType.NText); myParameter.Value = content; cmd.Parameters.Add(myParameter); try { connection.Open(); count = cmd.ExecuteNonQuery(); } catch (SqlException E) { throw new Exception(E.Message); } finally { cmd.Dispose(); connection.Close(); } } return count; } public static int ExecuteSql(string SQLString, params SqlParameter[] cmdParms) { int count; using (SqlConnection connection = new SqlConnection(connectionString)) { using (SqlCommand cmd = new SqlCommand()) { try { PrepareCommand(cmd, connection, null, SQLString, cmdParms); int rows = cmd.ExecuteNonQuery(); cmd.Parameters.Clear(); return rows; } catch (SqlException E) { throw new Exception(E.Message); } } } return count; } public static int ExecuteSqlByTime(string SQLString, int Times) { int count; using (SqlConnection connection = new SqlConnection(connectionString)) { using (SqlCommand cmd = new SqlCommand(SQLString, connection)) { try { connection.Open(); cmd.CommandTimeout = Times; return cmd.ExecuteNonQuery(); } catch (SqlException E) { connection.Close(); throw new Exception(E.Message); } } } return count; } public static object ExecuteSqlGet(string SQLString, string content) { object esg; using (SqlConnection connection = new SqlConnection(connectionString)) { SqlCommand cmd = new SqlCommand(SQLString, connection); SqlParameter myParameter = new SqlParameter("@content", SqlDbType.NText); myParameter.Value = content; cmd.Parameters.Add(myParameter); try { connection.Open(); object obj = cmd.ExecuteScalar(); if (object.Equals(obj, null) || object.Equals(obj, DBNull.Value)) { return null; } esg = obj; } catch (SqlException E) { throw new Exception(E.Message); } finally { cmd.Dispose(); connection.Close(); } } return esg; } public static int ExecuteSqlInsertImg(string strSQL, byte[] fs) { int count; using (SqlConnection connection = new SqlConnection(connectionString)) { SqlCommand cmd = new SqlCommand(strSQL, connection); SqlParameter myParameter = new SqlParameter("@fs", SqlDbType.Image); myParameter.Value = fs; cmd.Parameters.Add(myParameter); try { connection.Open(); count = cmd.ExecuteNonQuery(); } catch (SqlException E) { throw new Exception(E.Message); } finally { cmd.Dispose(); connection.Close(); } } return count; } public static void ExecuteSqlTran(ArrayList SQLStringList) { using (SqlConnection conn = new SqlConnection(connectionString)) { conn.Open(); SqlCommand cmd = new SqlCommand(); cmd.Connection = conn; SqlTransaction tx = conn.BeginTransaction(); cmd.Transaction = tx; try { for (int n = 0; n < SQLStringList.Count; n++) { string strsql = SQLStringList[n].ToString(); if (strsql.Trim().Length > 1) { cmd.CommandText = strsql; cmd.ExecuteNonQuery(); } } tx.Commit(); } catch (SqlException E) { tx.Rollback(); throw new Exception(E.Message); } } } public static void ExecuteSqlTran(Hashtable SQLStringList) { using (SqlConnection conn = new SqlConnection(connectionString)) { conn.Open(); using (SqlTransaction trans = conn.BeginTransaction()) { SqlCommand cmd = new SqlCommand(); try { foreach (DictionaryEntry myDE in SQLStringList) { string cmdText = myDE.Key.ToString(); SqlParameter[] cmdParms = (SqlParameter[])myDE.Value; PrepareCommand(cmd, conn, trans, cmdText, cmdParms); int val = cmd.ExecuteNonQuery(); cmd.Parameters.Clear(); trans.Commit(); } } catch { trans.Rollback(); throw; } } } } public static bool Exists(string strSql) { int cmdresult; object obj = GetSingle(strSql); if (object.Equals(obj, null) || object.Equals(obj, DBNull.Value)) { cmdresult = 0; } else { cmdresult = int.Parse(obj.ToString()); } if (cmdresult == 0) { return false; } return true; } public static bool Exists(string strSql, params SqlParameter[] cmdParms) { int cmdresult; object obj = GetSingle(strSql, cmdParms); if (object.Equals(obj, null) || object.Equals(obj, DBNull.Value)) { cmdresult = 0; } else { cmdresult = int.Parse(obj.ToString()); } if (cmdresult == 0) { return false; } return true; } public static int GetMaxID(string FieldName, string TableName) { object obj = GetSingle("select max(" + FieldName + ")+1 from " + TableName); if (obj == null) { return 1; } return int.Parse(obj.ToString()); } public static object GetSingle(string SQLString) { object gs; using (SqlConnection connection = new SqlConnection(connectionString)) { using (SqlCommand cmd = new SqlCommand(SQLString, connection)) { try { connection.Open(); object obj = cmd.ExecuteScalar(); if (object.Equals(obj, null) || object.Equals(obj, DBNull.Value)) { return null; } return obj; } catch (SqlException e) { connection.Close(); throw new Exception(e.Message); } } } return gs; } public static object GetSingle(string SQLString, params SqlParameter[] cmdParms) { object gs; using (SqlConnection connection = new SqlConnection(connectionString)) { using (SqlCommand cmd = new SqlCommand()) { try { PrepareCommand(cmd, connection, null, SQLString, cmdParms); object obj = cmd.ExecuteScalar(); cmd.Parameters.Clear(); if (object.Equals(obj, null) || object.Equals(obj, DBNull.Value)) { return null; } return obj; } catch (SqlException e) { throw new Exception(e.Message); } } } return gs; } private static void PrepareCommand(SqlCommand cmd, SqlConnection conn, SqlTransaction trans, string cmdText, SqlParameter[] cmdParms) { if (conn.State != ConnectionState.Open) { conn.Open(); } cmd.Connection = conn; cmd.CommandText = cmdText; if (trans != null) { cmd.Transaction = trans; } cmd.CommandType = CommandType.Text; if (cmdParms != null) { foreach (SqlParameter parameter in cmdParms) { if (((parameter.Direction == ParameterDirection.InputOutput) || (parameter.Direction == ParameterDirection.Input)) && (parameter.Value == null)) { parameter.Value = DBNull.Value; } cmd.Parameters.Add(parameter); } } } public static DataSet Query(string SQLString) { using (SqlConnection connection = new SqlConnection(connectionString)) { DataSet ds = new DataSet(); try { connection.Open(); new SqlDataAdapter(SQLString, connection).Fill(ds, "ds"); } catch (SqlException ex) { throw new Exception(ex.Message); } return ds; } } public static DataSet Query(string SQLString,int pageindx,int pagesize,string tablename) { using (SqlConnection connection = new SqlConnection(connectionString)) { DataSet ds = new DataSet(); try { connection.Open(); new SqlDataAdapter(SQLString, connection).Fill(ds,pageindx,pagesize,tablename); } catch (SqlException ex) { throw new Exception(ex.Message); } return ds; } } public static DataSet Query(string SQLString, int Times) { using (SqlConnection connection = new SqlConnection(connectionString)) { DataSet ds = new DataSet(); try { connection.Open(); SqlDataAdapter command = new SqlDataAdapter(SQLString, connection); command.SelectCommand.CommandTimeout = Times; command.Fill(ds, "ds"); } catch (SqlException ex) { throw new Exception(ex.Message); } return ds; } } public static DataSet Query(string SQLString, params SqlParameter[] cmdParms) { DataSet dsq; using (SqlConnection connection = new SqlConnection(connectionString)) { SqlCommand cmd = new SqlCommand(); PrepareCommand(cmd, connection, null, SQLString, cmdParms); using (SqlDataAdapter da = new SqlDataAdapter(cmd)) { DataSet ds = new DataSet(); try { da.Fill(ds, "ds"); cmd.Parameters.Clear(); } catch (SqlException ex) { throw new Exception(ex.Message); } dsq = ds; } } return dsq; } public static DataSet Query(string SQLString,string tablename) { using (SqlConnection connection = new SqlConnection(connectionString)) { DataSet ds = new DataSet(); try { connection.Open(); new SqlDataAdapter(SQLString, connection).Fill(ds, tablename); } catch (SqlException ex) { throw new Exception(ex.Message); } return ds; } } public static SqlDataReader RunProcedure(string storedProcName, IDataParameter[] parameters) { SqlConnection connection = new SqlConnection(connectionString); connection.Open(); SqlCommand command = BuildQueryCommand(connection, storedProcName, parameters); command.CommandType = CommandType.StoredProcedure; return command.ExecuteReader(); } public static int RunProcedure(string storedProcName, IDataParameter[] parameters, out int rowsAffected) { using (SqlConnection connection = new SqlConnection(connectionString)) { connection.Open(); SqlCommand command = BuildIntCommand(connection, storedProcName, parameters); rowsAffected = command.ExecuteNonQuery(); int result = (int)command.Parameters["ReturnValue"].Value; connection.Close(); return result; } } public static DataSet RunProcedure(string storedProcName, IDataParameter[] parameters, string tableName) { using (SqlConnection connection = new SqlConnection(connectionString)) { DataSet dataSet = new DataSet(); connection.Open(); SqlDataAdapter sqlDA = new SqlDataAdapter(); sqlDA.SelectCommand = BuildQueryCommand(connection, storedProcName, parameters); sqlDA.Fill(dataSet, tableName); connection.Close(); return dataSet; } } public static DataSet RunProcedure(string storedProcName, IDataParameter[] parameters, string tableName, int Times) { using (SqlConnection connection = new SqlConnection(connectionString)) { DataSet dataSet = new DataSet(); connection.Open(); SqlDataAdapter sqlDA = new SqlDataAdapter(); sqlDA.SelectCommand = BuildQueryCommand(connection, storedProcName, parameters); sqlDA.SelectCommand.CommandTimeout = Times; sqlDA.Fill(dataSet, tableName); connection.Close(); return dataSet; } } public static void RunStatProcedure(string storedProcName, IDataParameter[] parameters, out int rowsAffected) { using (SqlConnection connection = new SqlConnection(connectionString)) { connection.Open(); SqlCommand command = BuildIntCommand(connection, storedProcName, parameters); rowsAffected = command.ExecuteNonQuery(); connection.Close(); } } } }
最新发布
07-25
using System; using System.Data; using System.Data.SQLite; using System.IO; namespace SqliteTimeFilter { internal class Program { private const string InputDb = @"input.db"; private const string OutputDb = @"output.db"; private static void Main() { if (File.Exists(OutputDb)) File.Delete(OutputDb); SQLiteConnection.CreateFile(OutputDb); using var srcConn = new SQLiteConnection($"Data Source={InputDb}"); using var dstConn = new SQLiteConnection($"Data Source={OutputDb}"); srcConn.Open(); dstConn.Open(); // 1. 仅创建 _3CH_12 表结构 const string getCreateSql = "SELECT sql FROM sqlite_master WHERE type='table' AND name='_3CH_12'"; string createTableSql; using (var cmdGet = new SQLiteCommand(getCreateSql, srcConn)) { createTableSql = cmdGet.ExecuteScalar() as string; } if (string.IsNullOrEmpty(createTableSql)) throw new Exception("源数据库里没有 _3CH_12 表!"); using (var cmdCreate = new SQLiteCommand(createTableSql, dstConn)) cmdCreate.ExecuteNonQuery(); // 2. 查询 18:30:50 – 18:10:00 之间的数据 const string sqlSelect = @" SELECT id, time, father, child, cycle, current_sys, master, data FROM _3CH_12 WHERE datetime(substr(replace(time, '\"', ''), 1, 19)) BETWEEN datetime('2025-07-12 17:30:00') AND datetime('2025-07-12 18:30:00'); "; using var cmdSelect = new SQLiteCommand(sqlSelect, srcConn); using var reader = cmdSelect.ExecuteReader(); // 3. 插入到新库 const string sqlInsert = "INSERT INTO `_3CH_12`(id, time, father, child, cycle, current_sys, master, data) " + "VALUES(@id, @time, @father, @child, @cycle, @current_sys, @master, @data)"; using var cmdInsert = new SQLiteCommand(sqlInsert, dstConn); cmdInsert.Parameters.Add("@id", DbType.Int64); cmdInsert.Parameters.Add("@time", DbType.String); cmdInsert.Parameters.Add("@father", DbType.String); cmdInsert.Parameters.Add("@child", DbType.String); cmdInsert.Parameters.Add("@cycle", DbType.String); cmdInsert.Parameters.Add("@current_sys", DbType.Int32); cmdInsert.Parameters.Add("@master", DbType.String); cmdInsert.Parameters.Add("@data", DbType.Binary); int count = 0; while (reader.Read()) { cmdInsert.Parameters["@id"].Value = reader["id"]; cmdInsert.Parameters["@time"].Value = reader["time"]; cmdInsert.Parameters["@father"].Value = reader["father"]; cmdInsert.Parameters["@child"].Value = reader["child"]; cmdInsert.Parameters["@cycle"].Value = reader["cycle"]; cmdInsert.Parameters["@current_sys"].Value = reader["current_sys"] == DBNull.Value ? DBNull.Value : reader["current_sys"]; cmdInsert.Parameters["@master"].Value = reader["master"]; cmdInsert.Parameters["@data"].Value = reader["data"]; cmdInsert.ExecuteNonQuery(); count++; } Console.WriteLine($"Finished. 共写入 {count} 条记录到 {OutputDb}"); } } }这是全部代码,怎么修改
07-21
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