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Update 2 million rows for 1 column

I have a table with approximately 2 million records. I have to loop through each record and update the effective date. I need to set the day to the first of the month for each date.
If the current date is the first of the month, then ignore.
i.e.
07/01/2018
07/21/2018 => 07/01/2018
08/11/2018 => 08/01/2018
Currently, I'm writing this as a C# program and it taking way too long.
Is there a better solution?

Just use DATEADD() and DATEDIFF() combination to get the first of the month date
UPDATE t
SET datecol = DATEADD(MONTH, DATEDIFF(MONTH, '1900-01-01', datecol), '1900-01-01')
FROM yourtable t;

It could be as simple as:
Update myTable
set myDate = DateAdd(day, 1-Day(myDate), myDate)
where day(myDate) > 1;

Using sql to add to dates during a query

I have a table that keeps track of when particular events occur, and how long they last. For reasons I cannot fathom, it was designed to store the start date of the event, start time of the event, then the number of hours and minutes the event lasted. Like this:
EventStartDate | EventStartTime | TimeSpentHours | TimeSpentMinutes
Where EventStartDate is a dateTime with the hours/minutes always set to zero, so that, even though it's a date time, all the values are like "12/22/2016 00:00". The EventStartTime is a char(4) which is military time of the start of the event. TimeSpentHours is and int which is the total hours the event duration, and TimeSpentMinutes is an int for the number of minutes. Obviously the total time spent for the event is the hours plus the minutes.
The problem: I need to be able to, given a particular DateTime, find all the events that were occuring during that time. Put another way, given a particular DateTime I need to get all the events with a starting date and time that's greater than or equal to the given DateTime and less than or equal to an "end" date and time.
So I need to compute the "EndDateTime" based off the values in the database during the query. The database is SqlServer 2008 R2. I am using C# for WinForm application to query the data.
So far I have roughly:
public static List<ImportantEvents> GetEventsDuringDateTime(DateTime timeOfEvent)
{
using (SqlConnection sqlConn = getAndOpenSqlConn())
{
string theEventTime = timeOfEvent.ToString("hhmm");
string sqlStmt = "SELECT EVENT_ID, AGENCY, EVENTSTARTDATE, ACTNOTE, EVENTSTARTTIME, TIMESPENTHOURS, TIMESPENTMINUTES FROM EVENTSMAIN WHERE((EVENTSTARTDATE<= #MYEVENTDATETIME AND EVENTSTART TIME< #ACTTIME) AND ...";"
}
}
(the above SQL obviously won't work and is where I am stuck...)
My question is: how can I, in the query, add the EVENTSTARTTIME to the EVENTSTARTDATE to create a new "temporary" column, then add the TIMESPENTHOURS and TIMESPENTMINUTES to that column into another new "temporary" column, to then query against given a specific DateTime value???

It is possible to achieve this in a single query with a common-table expression like this:
With StartAndEndTimes As (
Select Event_ID,
EventStart = DateAdd(Minute, Convert(int, Right(EventStartTime, 2)), DateAdd(Hour, Convert(int, Left(EventStartTime, 2)), EventStartDate)),
EventEnd = DateAdd(Minute, Convert(int, Right(EventStartTime, 2))+TimeSpentMinutes, DateAdd(Hour, Convert(int, Left(EventStartTime, 2))+TimeSpentHours, EventStartDate))
From EventsMain)
Select Event_Id, EventStart, EventEnd, <<add other fields here>>
From StartAndEndTimes
Where EventStart <= #MyEventDateTime
And EventEnd > #MyEventDateTime;
Basically you can extract the hours and minutes from the start time and add them to the start date to get a true, datetime, start date. Similar with the end date. It is not necessary to use common-table expression here, but it does make the code more readable. Then you just do the ordinary date comparison to your input parameter.

Here I have disected parts of the final query. You will need to put the final part into your query wherever you need it.
SELECT Combined = EVENTSTARTDATE + EVENTSTARTTIMEFROM FROM EventsMain
SELECT CombinedWithHour = DATEADD(hh, TIMESPENTHOURS, Combined) FROM EventsMain
SELECT CombinedWithMinute = DATEADD(mi, TIMESPENTMINUTES, CombinedWithHour) FROM EventsMain
All together:
SELECT DATEADD(mi, TIMESPENTMINUTES, DATEADD(hh, TIMESPENTHOURS, EVENTSTARTDATE + EVENTSTARTTIME)) FROM EventsMain

Search data only day value of date

Is it possible to search a data by day or month value of a date type value in mysql and c#.net.please can any one help me?

You can use DAY and MONTH functions:
SELECT *
FROM yourtable
WHERE DAY(date_column) = 1
or
SELECT *
FROM yourtable
WHERE MONTH(date_column) = 1
have a look at MySQL date and time functions.

How to update column in SQL Server 30 days from stored date on form load?

I want to take the rows with a createDate 30 days old and change the bit value IsExpired to 1. How do I accomplish this?
Here is the query I have tried:
UPDATE [mydb].[dbo].[mytable]
SET IsExpired = 1
WHERE CreateDate > (time, SYSDATETIME(GETDATE(CreateDate)+30))
The CreateDate column has a datetime string stored within it from a C# project. For example the value stored in the first row is 2013-05-29 14:59:48.000. When I execute it in SQL I get an error
Msg 102, Level 15, State 1, Line 1
Incorrect syntax near ','.

Try something like this:
UPDATE [mydb].[dbo].[mytable]
SET IsExpired = 1
WHERE CreateDate < dateadd(dd, -30, getdate())

The following should match where the CreateDate field is less than 30 days prior to the current date of your SQL server. You may want to use GETUTCDATE() if you store your times in UTC.
UPDATE [mydb].[dbo].[mytable] SET IsExpired=1
WHERE CreateDate < DATEADD(DAY, -30, GETDATE())

Try below Query:
UPDATE [mydb].[dbo].[mytable] SET IsExpired=1 WHERE CreateDate < DATEADD(dd, -30, GETDATE())
Also this post will help you to know about DATEADD().
First I assumed that Skullomania is trying to get 30 days of CreateDate. Later from Scott Chamberlain comments, I got the point of user.

You can try:
UPDATE [mydb].[dbo].[mytable]
SET IsExpired=1
WHERE CreateDate = DATEADD(MONTH, -1, GETDATE())

Sql query with the current date

I've got a simple query where I want to put the current date
var query = #"
SELECT trainid, trainnum
FROM trains
WHERE CONVERT(varchar(10), trainstartdate, 104)=" +
" " +
// so that matches the '104' format
String.Format("{0:dd.MM.YYYY}", DateTime.Now) +
" " +
"ORDER BY trainnum";
But when running I get the error message:
Cannot call methods on numeric. .Net SqlClient Data Provider
How do I specify current date the right way?
Thanks!
Using GETDATE()
Effect:
The conversion of a varchar data type to a datetime data type resulted in an out-of-range value
Using {0:dd.MM.yyyy}
Effect: none
Using CONVERT(varchar(20), GetDate(), 104)
Effect: that works!
Thanks!

Description
I would not convert to a varchar and doing string comparrisson. The performance is much better if you compare trainstartdate using the >= and <.
You can use the T-SQL getDate() method to get the current date.
getDate() returns the current datetime with the time. 2012-02-14 14:51:08.350
DATEADD(dd, 0, DATEDIFF(dd, 0, GETDATE())) return only the current date. `2012-02-14 00:00:00.000
DATEADD(dd, 1, DATEDIFF(dd, 0, GETDATE())) returns only the date of tomorow. 2012-02-15 00:00:00.000
Sample
var query = #"
SELECT trainid, trainnum
FROM trains
WHERE trainstartdate >=
-- today
DATEADD(dd, 0, DATEDIFF(dd, 0, GETDATE()))
AND trainstartdate <
-- tommorow
DATEADD(dd, 1, DATEDIFF(dd, 0, GETDATE()))
ORDER BY trainnum"
Note:
If you want to be ANSI compliant, CURRENT_TIMESTAMP does the same.
More Information
MSDN - GETDATE (Transact-SQL)
MSDN - DATEDIFF (Transact-SQL)
MSDN - DATEADD (Transact-SQL)
Stackoverflow - CURRENT_TIMESTAMP vs GetDate()

var query = #"
SELECT trainid, trainnum
FROM trains
WHERE CONVERT(varchar(10), trainstartdate, 104)=
CONVERT(varchar(20), GetDate(), 104)
ORDER BY trainnum";

GETDATE() is all you need...

I think
String.Format("{0:dd.MM.YYYY}", DateTime.Now);
is returning the date with a dot, which makes SQL consider it as a number.
Try using
String.Format("{0:MM/dd/yyyy}", DateTime.Now);
with a / instead.

Change the format pattern of YYYY to small-case letters
{0:dd.MM.yyyy}

You need to be aware that GETDATE() returns the current date and time of day, not only today's date.
If you want to return rows matching today's date, you need to extract the date part. There are a number of ways to do this - e.g. with SQL Server 2008 you can use the DATE data type, but one general way that works with earlier versions of SQL Server is the following:
CONVERT(DATETIME, convert(VARCHAR(10),getdate(),101) )
You can then use the query:
SELECT trainid, trainnum
FROM trains
WHERE trainstartdate = CONVERT(DATETIME, convert(VARCHAR(10),getdate(),101) )
which will work provided you are sure that the date/time in the trains.trainstartdate column is a date only (time of day = 0).
If trainstartdate contains the start date/time, you can get all of today's trains as follows:
SELECT trainid, trainnum
FROM trains
WHERE trainstartdate >= CONVERT(DATETIME, convert(VARCHAR(10),getdate(),101) )
AND trainstartdate < DATEADD(dd,1, CONVERT(DATETIME, convert(VARCHAR(10),getdate(),101) ))
By doing it like this rather than converting to a string, you will take advantage of any index there may be on the trainstartdate column.

Try this .. YYYY should be small letter yyyy
String.Format("{0:dd.MM.yyyy}", DateTime.Now)