C# 如何使用dapper映射/连接两个表并按参数筛选

我想连接两个相互有关系的表， 在客户表中，有两列用于删除和批准。获取已批准但未删除的客户，然后使用CustId将客户与贷款表匹配（两个表都有CustId），并根据提供的参数进行筛选

• 已删除=0
• 批准=1

传递的参数是（并且可以是可选的）用户只能传递一个、两个或多个参数 利率 甘薯 最后付款日期

        public async Task<IEnumerable<Loan>> ManageGettingAll(string InterestRate, string LoanTrfDate, string LastPaymentDate)
        {
            IEnumerable<Loan> loans = null;

            //var sql = "Select * from MonthlyInterest where LoanAccountNo=@Id AND Deleted=@Deleted AND CAST(TranDate AS DATE) BETWEEN @StartDate AND @EndDate";
            try{

                using (var conn = new SqlConnection(connectionstring))
                {
                    await conn.OpenAsync();

                    var parameters = new 
                    { 
                        InterestRate = InterestRate, 
                        LoanTrfDate = LoanTrfDate, 
                        LastPaymentDate = LastPaymentDate
                    };
                   
                string sql = "Select l.*, c.FirstName_CompanyName from dbo.Loan l left join Customer where and l.Deleted = 0 and and l.Approved = 1 c on  l.CustId=c.CustId Where InterestRate=@InterestRate AND CAST(LoanTrfDate AS DATE)=@LoanTrfDate AND CAST(LastPaymentDate AS DATE)=@LastPaymentDate";
                 
                    loans = await conn.QueryAsync<Loan>(sql, parameters);
                }
            }
            catch (Exception ex)
            {

                throw;
            }


            return loans;
        }

public async Task ManageGettingAll（字符串InterestRate、字符串LoanTrfDate、字符串LastPaymentDate）
{
IEnumerable loans=null；
//var sql=“Select*from MonthlyInterest，其中LoanAccountNo=@Id和Deleted=@Deleted并在@StartDate和@EndDate之间转换（TranDate为DATE）”；
试一试{
使用（var conn=newsqlconnection（connectionstring））
{
等待连接OpenAsync（）；
var参数=新
{ 
利率=利率，
LoanTrfDate=LoanTrfDate，
LastPaymentDate=LastPaymentDate
};
string sql=“Select l.*，c.FirstName\u CompanyName from dbo.Loan l left join Customer where and l.Deleted=0 and l.Approved=1 c on l.CustId=c.CustId where InterestRate=@InterestRate and CAST（LoanTrfDate）=@LoanTrfDate and CAST（LastPaymentDate）=@LastPaymentDate”；
loans=await conn.querysync（sql，参数）；
}
}
捕获（例外情况除外）
{
投掷；
}
归还贷款；
}

要仅在参数不为null时按参数进行筛选，可以使用下面的SQL

Select l.*, c.FirstName_CompanyName 
from dbo.Loan l 
left join Customer c on l.CustId=c.CustId 
where l.Deleted = 0 and l.Approved = 1
and (@InterestRate is null or InterestRate=@InterestRate) 
AND (@LastPaymentDate is null or (CAST(LoanTrfDate AS DATE)=@LoanTrfDate AND CAST(LastPaymentDate AS DATE)=@LastPaymentDate))

若要检查是否至少设置了一个参数，可以添加此if语句

if(string.IsNullOrWhiteSpace(LoanTrfDate) && string.IsNullOrWhiteSpace(LastPaymentDate))
{
    throw new Exception("Some clear message here.");
} 

---

需要明确的是，您的问题是如何仅在指定值时按参数过滤？并忽略其参数是否为空？必须至少传递一个参数。您可以使用DynamicParameters对象。您可以检查传递的字符串是否不为null，然后仅将这些字符串添加到DynamicParameters对象。在SQL端，您可以使用ISNULL（@InterestRate，InterestRate）或Coalease函数。