如何获得特定月份和年份的所有条目？

Question

I am trying to do

var dt = DateTime.Now.ToString();

string query = "SELECT * FROM Gigs where Date >= '"+dt+"';";

it appears to work - except it gets entries for future months

I then do

var dt = DateTime.Now.AddMonths(3).ToString();

string query = "SELECT * FROM Gigs where Date >= '"+dt+"';";

And I get the same result which isn't expected

If today is January 1st, I want to get all the entries for January, but none for February.

This is readonly data, no concern of SQL injection

Answer 1

First of all, why do you need to calculate date in C# and pass it to SQL query? You can use DATEADD and GETDATE() to avoid that:

--Will get three months before:
select DATEADD(MONTH, 3, GETDATE())

Second, you got three months in future. Are you sure that you don't want to use -3:

var dt = DateTime.Now.AddMonths(-3).ToString();

Third: use parameterized queries, here is the example:

using (var connection = new SqlConnection("Data Source=.;Initial Catalog=Test;Integrated Security=SSPI;"))
            {
                connection.Open();
                using (var command = connection.CreateCommand())
                {
                    command.CommandText = "SELECT *   FROM [Test].[dbo].[SomeDates] WHERE dt > @MyDate";
                    var param = new SqlParameter("@MyDate", SqlDbType.DateTime2) { Value = DateTime.Now };
                    command.Parameters.Add(param);
                    var result = command.ExecuteReader();
                    while (result.Read())
                    {
                        Console.WriteLine(result.GetDateTime(0));
                    }
                }
            }

Update regarding second : looks like you do want to get query in future.

Update #2 : Even if you is not concerned about SQL injection (but you must be), you should use parameterized queries since framework will take care of date time formats and conversions. Otherwise it may work on your machine but won't work on other machine/production.

Answer 2

If you are going to convert the date to a string, make sure your region settings don't return a different date for sql server if it uses different region settings. June 5th can be 05-06-2015 for you, but if you pass that in sql-server with en-us it will interpret it as May 6th.

Conversion from date to string for sql server should always be done using the universal format yyyy-MM-dd, which won't be interpreted wrong in most software, regardless of regional settings.

If you use SqlParameters as suggested you don't really need to worry about these mismatches, so another + for SqlParameter usage.