//C# 代码
int year = DateTime.Now.Year;
int month = DateTime.Now.Month;
DateTime firstDayOfThisMonth = new DateTime(year, month, 1);
DateTime lastDayOfThisMonth = new DateTime(year, month, DateTime.DaysInMonth(year, month));

//SQL代码
DECLARE @NOW DATETIME
DECLARE @FistDayOfThisMonth DATETIME 
DECLARE @LastDayOfThisMonth DATETIME
SET @NOW = GETDATE()
SET @FistDayOfThisMonth = @NOW - DAY(@NOW) + 1 
SET @LastDayOfThisMonth = DATEADD(MONTH, 1, @NOW - DAY(@NOW) + 1) - 1
SELECT @FistDayOfThisMonth
SELECT @LastDayOfThisMonth