How can I select the first day of a month in SQL?

Question

How can one select the first day of the month of a given DateTime variable?

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I know it's quite easy to do using this kind of code:

select CAST(CAST(YEAR(@mydate) AS VARCHAR(4)) 
+ '/' + CAST(MONTH(@mydate) AS VARCHAR(2)) + '/01' AS DATETIME)

This is not very elegant, and not very fast either.

Is there a better way to do this?

Answer 1

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