/// <summary>
/// 查询一个值
/// </summary>
/// <typeparam name="T"></typeparam>
/// <typeparam name="TValue"></typeparam>
/// <param name="field"></param>
/// <param name="condition"></param>
/// <returns></returns>
public static TValue ExecuteScalar <T, TValue>(this DbExecutor db, Expression <Func <T, TValue> > field, Expression <Func <T, bool> > condition) where T : class
{
using (ExpressionCondition expression = new ExpressionCondition())
{
expression.Visit(field);
string sql = $"SELECT TOP 1 {expression.ToString()} FROM [{typeof(T).GetTableName()}]";
expression.Visit(condition);
sql += expression.ToCondition(out DynamicParameters parameters);
object value = db.ExecuteScalar(CommandType.Text, sql, parameters);
if (value == null)
{
return(default);
