SQL statements about time query small ideas, I hope you give some advice

Query this month's information:
Select * from T_users Where DateDiff (Month,registertime,getdate ()) =0

Yesterday's message:
SELECT * from T_users where Lastlogintime>dateadd (Day,-2,getdate ()) and Lastlogintime<getdate ()

Selectsum ([Detailmoney]) from [dbo]. [T_buydetails] INNER JOIN [dbo]. [T_users] On [Commenderid]=[userid]
WHERE DateDiff (Month,[datetime],getdate ()) =0

--This month

Selectsum ([Detailmoney]) from [dbo]. [T_buydetails]
INNER JOIN [dbo]. [T_users] On [Commenderid]=[userid]
WHERE CONVERT (char), [datetime],120] >convert (char (10), DATEADD (Day,-2,getdate ()), and (char) and CONVERT (chars), [ datetime],120) <convert (char), GETDATE (), 120)

--Yesterday

SELECT SUM ([Detailmoney]) from [dbo]. [T_buydetails]
WHERE CONVERT (char), [datetime],120] =convert (char (Ten), GETDATE (), 120)

--Today

SELECT SUM ([Detailmoney]) from [dbo]. [T_buydetails]

--Total consumption