How to Efficiently Select the First Day of a Month in SQL?

Efficiently select the first day of the month in SQL

Retrieving the first day of the month from a given DateTime variable is a common task in database operations. While code using the CAST function is a possible solution, it's not the most efficient or elegant approach.

Better solution

To select the first day of the month, a more efficient way is to use the DATEADD and DATEDIFF functions. The following code demonstrates this technique:

<code class="language-sql">SELECT DATEADD(month, DATEDIFF(month, 0, @mydate), 0) AS StartOfMonth</code>

Instructions:

• DATEDIFF(month, 0, @mydate) Calculates the difference between the given DateTime variable @mydate and the start of the current month (i.e. January 1st).
• DATEADD(month, DATEDIFF(month, 0, @mydate), 0) Add the calculated difference back to the beginning of the current month to get the @mydate first day of the month.

This method is faster and more concise than the previous method using the CAST function.

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