Understanding SQL Querying for Weekly Data: Mastering Date-Related Functions to Avoid Overlapping Year Dates

Understanding SQL Querying for Weekly Data

In this article, we will delve into the intricacies of querying weekly data using SQL. Specifically, we’ll explore how to group data by weeks of the year, avoiding overlapping year dates. We’ll also examine the differences between various date-related functions in SQL and provide examples to illustrate our points.

Before we dive into the nitty-gritty of querying weekly data, let’s briefly discuss some key date-related functions that you should be familiar with:

DATEPART: Returns a value representing a specific part of a date. For example, DATEPART(WEEK, responsedate) returns the week number for a given date.
YEAR: Returns the year in which the date falls. However, this can lead to issues when dealing with weeks that span multiple years.
ISOYEAR: This function returns the ISO year of the date, taking into account the week numbering system defined by the International Organization for Standardization (ISO).

The Problem: Overlapping Year Dates

When using DATEPART(WEEK, responsedate) to group data by weeks of the year, you might encounter overlapping year dates. For instance, consider a date like 01/01/23, which falls on a Sunday and belongs to the last week of 2022.

{< highlight sql >}
SELECT DATEPART(WEEK, responsedate) AS week_number,
       responsedate
FROM survey
WHERE EXTRACT(YEAR FROM responsedate) = EXTRACT(YEAR FROM GETDATE())
AND responsedate <= GETDATE()
ORDER BY responsedate;
{< /highlight >}

In this query, the date 01/01/23 is included in the results because EXTRACT(YEAR FROM responsedate) equals the current year. However, this can lead to inaccurate grouping of data.

The Solution: Using ISOYEAR and WEEK

To avoid overlapping year dates, you should use both ISOYEAR and WEEK. Here’s an updated query:

{< highlight sql >}
SELECT DATEPART(WEEK, responsedate) AS week_number,
       CAST(MIN(responsedate) AS DATE) AS from_date,
       CAST(MAX(responsedate) AS DATE) AS to_date,
       (COUNT(CASE WHEN ov_rating >= 8 THEN 1 END) * 100.0 / NULLIF(COUNT(ov_rating), 0)) AS overall_rating,
       (COUNT(CASE WHEN recommend >= 9 THEN 1 END) * 100.0 / NULLIF(COUNT(recommend), 0)) -
       (COUNT(CASE WHEN recommend BETWEEN 1 AND 6 THEN 1 END) * 100.0 / NULLIF(COUNT(recommend), 0)) AS recommend_score
FROM survey
WHERE EXTRACT(ISOYEAR FROM responsedate) = EXTRACT(ISOYEAR FROM GETDATE())
AND responsedate <= GETDATE()
GROUP BY DATEPART(WEEK, responsedate)
ORDER BY from_date;
{< /highlight >}

In this query:

EXTRACT(ISOYEAR FROM responsedate) ensures that you’re using the ISO year of the date.
DATEPART(WEEK, responsedate) returns the week number for each date.

By combining these two functions, you can ensure accurate grouping of data by weeks of the year, avoiding overlapping year dates.

Example Use Cases

Here are some additional examples that demonstrate how to use ISOYEAR and WEEK in different contexts:

Example 1: Grouping Data by Week

Suppose you want to group your data by week, with each week spanning Monday to Sunday. In this case, you can use the following query:

{< highlight sql >}
SELECT DATEPART(WEEK, responsedate) AS week_number,
       CAST(MIN(responsedate) AS DATE) AS from_date,
       CAST(MAX(responsedate) AS DATE) AS to_date,
       (COUNT(CASE WHEN ov_rating >= 8 THEN 1 END) * 100.0 / NULLIF(COUNT(ov_rating), 0)) AS overall_rating,
       (COUNT(CASE WHEN recommend >= 9 THEN 1 END) * 100.0 / NULLIF(COUNT(recommend), 0)) -
       (COUNT(CASE WHEN recommend BETWEEN 1 AND 6 THEN 1 END) * 100.0 / NULLIF(COUNT(recommend), 0)) AS recommend_score
FROM survey
WHERE EXTRACT(ISOYEAR FROM responsedate) = EXTRACT(ISOYEAR FROM GETDATE())
AND responsedate <= GETDATE()
GROUP BY DATEPART(WEEK, responsedate)
ORDER BY from_date;
{< /highlight >}

Example 2: Calculating Weekly Scores

Suppose you want to calculate weekly scores for your survey respondents. In this case, you can use the following query:

{< highlight sql >}
SELECT DATEPART(WEEK, responsedate) AS week_number,
       CAST(MIN(responsedate) AS DATE) AS from_date,
       CAST(MAX(responsedate) AS DATE) AS to_date,
       (COUNT(CASE WHEN ov_rating >= 8 THEN 1 END) * 100.0 / NULLIF(COUNT(ov_rating), 0)) AS overall_rating,
       (COUNT(CASE WHEN recommend >= 9 THEN 1 END) * 100.0 / NULLIF(COUNT(recommend), 0)) -
       (COUNT(CASE WHEN recommend BETWEEN 1 AND 6 THEN 1 END) * 100.0 / NULLIF(COUNT(recommend), 0)) AS recommend_score
FROM survey
WHERE EXTRACT(ISOYEAR FROM responsedate) = EXTRACT(ISOYEAR FROM GETDATE())
AND responsedate <= GETDATE()
GROUP BY DATEPART(WEEK, responsedate)
ORDER BY from_date;
{< /highlight >}

In these examples:

EXTRACT(ISOYEAR FROM responsedate) ensures that you’re using the correct ISO year.
DATEPART(WEEK, responsedate) returns the week number for each date.

By combining these functions, you can ensure accurate calculations of weekly scores and grouping of data by weeks of the year.

Last modified on 2025-01-04