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C# SQL Server compare date with string

I have to compare a table field that is a date stored as varchar in the format 'dd/MM/yyyy' with a date value, but the comparison fails. I have exception
Conversion failed when converting date and/or time from character string.
I tried converting the date to compare i nstring, like this
string dateFormat = date.ToString("dd/MM/yyyy");
and then write the query like this:
string sql = "select * from TB_RICHIESTE where CONVERT(DATE, Date) <= CONVERT(DATE, '" + dateFormat + "')";
But I have this excpetion. Someone can help me? Thanks

First, you should not store dates as strings.
As Panagiotis Kanavos wrote in his comment - this is a serious bug. You can't sort by such a column, you can't search for date ranges, and most important - you can't control if someone enters an invalid value - nothing is stopping someone from entering "Alfredo" to that column.
For more information, read Aaron Bertrand's Bad habits to kick : choosing the wrong data type.
Second, you should not pass dates from .Net to Sql server as strings. you should pass instances of DateTime as parameters.
The .Net DateTime maps directly to SQL Server's Date.
If you can't change the data type of the column, you can at least convert it to date using the proper convert style (103 in your case).
Here is a better way to do it:
var sql = "select * from TB_RICHIESTE where CONVERT(DATE, [Date], 103) <= #Date";
Then you add the #Date parameter to the SqlCommand:
com.Parameters.Add("#Date", SqlDbType.Date).Value = date.Date;

Use Parameter to pass date values refer #Zohar Peled post. This is the proper method handling date values.
OR
You can pass the date value in ISO format, refer the below code.
string dateFormat = date.ToString("yyyy/MM/dd");
string sql = "select * from TB_RICHIESTE where CONVERT(DATE, Date) <= CONVERT(DATE, '" + dateFormat + "')";

datetime query in SQL

I have to write a query to get rows where date is like current date.
I don't want to compare the the time part but only date part.
Like today's date is 2014-05-03
but in table its in datetime as 2014-05-03 10:08:22
i tried [http://www.w3schools.com/sql/func_convert.asp]
but could not do anything..
my query is like
select *from dbo.param where cvdatetime like '2014-05-03%';
but its does not work although if i use
select *from dbo.param where cvdatetime like '%2014%';
it works so i don't get why "like" can't work in the previous case
i just want to compare the date part only not the time part..
like in c# i will take the current date as
string today_n = DateTime.Now.ToShortDateString();
which will give only today's date
the query will be like
string query="select *from dbo.param where cvdatetime like '" + today_n + "%'"
what is the correct way?
also i want that whatever be the system date format query should work like even if system date time format is dd-mm-yyyy hh:mm:ss tt the query should work how can i ensure this?
Adding new requirement what if I need to check date hh:mm: only not seconds
i.e, 2014-05-04 12:00: part only not seconds part

You cannot use like as such on datetime column.
Use the below:
select * from dbo.param where convert(varchar, cvdatetime, 120) like '%2014%';
As, far your second question is concerned, you'll have to use parameterized queries to avoid sql injection attacks.

Using SQL Server 2005 or later, just convert the datetime to date:
select *
from dbo.param
where cast(cvdatetime as date) = '2014-05-03';
Do not think about dates as strings. Think of them as dates, with the string format only used for output purposes. Or, if you have to make an analogy to another type, think numbers. As an example, go into Excel, put a date into a cell. Nicely format it. Then set another cell equal to the value of the first cell (=A1 for example). Format that as a number and you will see some strange number, probably in the range of about 40,000. Excel stores dates as number of days since 1970-01-01. SQL has a similar (but different) storage mechanism.

You can use the following for the current date:
SELECT *
FROM dbo.param
WHERE CONVERT(DATE, cvdatetime) = CONVERT(DATE, GETDATE())

How to use DateTime with SQL queries?

How should I be using c# datetime values in my C# queries?
Up until now I've been doing the following:
Datetime my_time = new DateTime(2012, 09, 05);
cmd.Parameters.AddWithValue("#date", my_time);
cmd.CommandText = "SELECT * FROM sales WHERE date = #date";
Which has been working fine until this morning when I've been getting null errors when retrieving values from the results set.
I changed my date parameter to grab the short date string from my datetime value like this:
cmd.Parameters.AddWithValue("#date", my_time.ToShortDateString());
And it appears to be working fine again.
What is the correct way of doing this? Obviously I'm converting my datetime value to a string, only (presumably) for the database to convert it back to a datetime before evaluating my equality where clause, which seems a little pointless.
Edit:
I'm using SQL Server Compact Edition and the date field in my example has a precision setting of 8 (date, excluding time right?). Does that mean I was passing the full date with 00:00:00.000 on the end before? How might I pass a datetime value to the database without including the time, without converting to a string first?

You can just try with my_time.Date
cmd.Parameters.AddWithValue("#date", my_time.Date);

Do not pass it as a string or will run in troubles with culture formatting.
Try using SqlDateTime instead.

You're not converting the date to a string - AddWithValue knows it is a date, but it is a datetime, so it has a time component. Do your sales have a time component to them?
SELECT * FROM sales WHERE DateDiff(d, #date, date) = 0

DATE datatype of SQL Server 2008 returing time!

I have a column of datatype DATE in SQL Server 2008, which stores only the date in format "yyyy-MM-dd" but when I select the date from database via C# and converting into string, it shows the time part also like.
"2012-04-21 12:00:00"
what is the problem I didn't store time in database here is my query!
string sql = #"select card_no, amount, csc,expdate " +
"from user_account_card " +
"where user_id = '" + loginsession.Auser + "';";
SqlCommand cmd = new SqlCommand(sql, con);
SqlDataReader red = cmd.ExecuteReader();
while (red.Read())
{
loginsession.Cardno = Convert.ToString(red["card_no"]);
loginsession.Cardamount = Convert.ToString(red["amount"]);
loginsession.Csc=Convert.ToString(red["csc"]);
loginsession.Expdate = Convert.ToString(red["expdate"]);//date part
break;
}
MessageBox.Show("database value from database--" +loginsession.Expdate);
Please help me what to do

If the time is showing as midnight then for all intents and purposes it is storing it without time.
SQL has a date data type that does not include time, but you need to re-design the table. Not sure if you need/want to do that.
You could also try this:
((Date)red["expdate"]).ToString();
Since this will convert to a Date data type and not a DateTime data type you should just see the date part in the returned string.

Convert.ToString doesn't have a "date" overload, only "datetime"

SQL Server 2008 will store just date in a DATE column - you can easily verify that in SQL Server Management Studio. Also: a DATE column doesn't store a date in a particular string format - that's just how you see it. A date is a date is a date - it's stored as a date and doesn't have any "format" per se.
However, .NET doesn't have a "date-only" datatype, so reading a DATE column from SQL Server ends up in a DateTime variable in .NET which contains DATE and TIME.
So it's really not an issue of SQL Server here - but rather an issue in .NET

c# query is not returning anything

here is my query:
select reporttime, datapath, finalconc, instrument from batchinfo
join qvalues on batchinfo.rowid=qvalues.rowid where qvalues.rowid
in (select rowid from batchinfo where instrument LIKE '%TF1%' and reporttime
like '10/%/2010%') and compound='ETG' and name='QC1'
i am running it like this:
// Create a database connection object using the connection string
OleDbConnection myConnection = new OleDbConnection(myConnectionString);
// Create a database command on the connection using query
OleDbCommand myCommand = new OleDbCommand(mySelectQuery, myConnection);
it does not return any results.
when i try this same query in sql server GUI it returns lots of rows
is there a problem specifically with the syntax of the query for c#?
please note that if i simplify the query like select * from table, there is no issue

Can you try using SqlCommand instead of OleDbCommand?
According to MSDN it: Represents a Transact-SQL statement or stored procedure to execute against a SQL Server database.
So if you really are using SQL Server 2008 you should probably use this. Any reason you are not?

How are you setting up the mySelectQuery string?
Could this be a problem with an escape character?
If you're not doing it already,
string mySelectQuery = #"query text here";

Personally I would run query profiler to see what query (if any) is being run, and then go from there.

Are you sure you are connecting to the same database in your code?
On a side note do you need the inner select? Couldn't you write this query as follows.
select reporttime,
datapath,
finalconc,
instrument
from batchinfo
join qvalues on batchinfo.rowid = qvalues.rowid
where compound = 'ETG'
and name = 'QC1'
and batchinfo.instrument like '%TF1%'
and batchinfo.reporttime like '10/%/2010%'

Edit - Never mind, just read the comment that your date is in a varchar field. Will leave this here since it may still be useful information.
Try this:
reporttime like 'Oct%2010%'
I have a test table here and when I query it using
where LastModified like '11/%/2010%'
no rows are returned, although all of the rows in the table have dates in November 2010. When I run the same query using like 'Nov%2010%', it returns all rows.
Details can be found here:
The LIKE clause can also be used to search for particular dates, as well. You need to remember that the LIKE clause is used to search character strings. Because of this the value which you are searching for will need to be represented in the format of an alphabetic date. The correct format to use is: MON DD YYYY HH:MM:SS.MMMAM, where MON is the month abbreviation, DD is the day, YYYY is the year, HH is hours, MM is minutes, SS is seconds, and MMM is milliseconds, and AM designates either AM or PM.