How Can I Efficiently Get the First Day of a Month in SQL?

Efficiently select the first day of the month in SQL

In SQL, getting the first day of a specified month is a common requirement. While the traditional approach of converting the year and month to strings and then appending "/01" (as shown below) works, it lacks efficiency and elegance:

<code class="language-sql">SELECT CAST(CAST(YEAR(@mydate) AS VARCHAR(4)) 
+ '/' + CAST(MONTH(@mydate) AS VARCHAR(2)) + '/01' AS DATETIME)</code>

A more efficient alternative

A more efficient and flexible approach is to use a combination of the DATEADD() and DATEDIFF() functions:

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

This method does the following:

1. DATEDIFF(month, 0, @mydate) Calculates the number of months between the specified date and midnight on January 1, 0001 (represented by 0 in the above expression). This actually determines the month number of the year.
2. DATEADD(month, DATEDIFF(month, 0, @mydate), 0) Increases the month number by the value calculated in the previous step, effectively resetting the date to the first day of the month.

This approach has significant performance advantages, especially when dealing with large data sets or complex queries.

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