我喜欢使用FORMAT，你甚至可以指定一个时间

SELECT FORMAT(@myDate,'yyyy-MM-01 06:00') first_of_a_month

当月的第一天和最后一天:

select dateadd(mm, -1,dateadd(dd, +1, eomonth(getdate()))) as FirstDay, 
eomonth(getdate()) as LastDay

SELECT DATEADD(month, DATEDIFF(month, 0, @mydate), 0) AS StartOfMonth

试试下面的方法:

select trunc(sysdate,'MM') 
from dual;

简单查询:

SELECT DATEADD(m, DATEDIFF(m, 0, GETDATE()), 0) 
-- Instead of GetDate you can put any date.

它可能相当快。为什么不创建一个sql函数呢?

CREATE FUNCTION [dbo].[GetFirstDayOfMonth] ( @InputDate    DATETIME )
RETURNS DATETIME
BEGIN

    RETURN CAST(CAST(YEAR(@InputDate) AS VARCHAR(4)) + '/' + 
                CAST(MONTH(@InputDate) AS VARCHAR(2)) + '/01' AS DATETIME)

END
GO