How Can I Reliably Extract Week Numbers from Dates in SQL?

Extracting Week Numbers from Dates

When working with date data in SQL, it's often necessary to extract information such as the week number. In some scenarios, however, attempting to retrieve the week number may result in null values. This article addresses how to overcome this issue and effectively extract week numbers from dates.

The issue arises when trying to extract the week number from a date column of varchar2 type. To resolve this, the date must first be converted to a true date datatype using the to_date function. However, this conversion alone may not yield the desired result.

To obtain the week number, it's necessary to convert the date back to varchar2 with the desired mask. This can be achieved using the following syntax:

to_char(to_date('date_string', 'format'), 'WW')

Where 'date_string' is the input date in varchar2 format, 'format' is the original date format, and 'WW' indicates the week number of the year.

If you prefer the week number as a number datatype, you can wrap the statement in to_number():

to_number(to_char(to_date('date_string', 'format'), 'WW'))

It's important to note that there are several week number options to consider, each with different calculations. The table below summarizes these options:

Parameter	Explanation
WW	Week of year (1-53)
W	Week of month (1-5)
IW	Week of year (1-52 or 1-53) based on the ISO standard

By understanding the appropriate conversion techniques and options, you can effectively extract week numbers from dates in SQL.

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