SQL Day of Week Functions Explained (With MySQL, PostgreSQL & SQL Server Examples)

Understanding how to work with days of the week in SQL is essential for many data analysis tasks, including SQL weekday interview problems and date manipulation coding challenges. This skill is frequently tested in SQL day-of-week technical screens, making it crucial for aspiring data analysts and business intelligence professionals.

SQL databases like MySQL, SQL Server, and PostgreSQL offer distinct functions to extract the day of the week from a date. These functions typically return either a number (1–7 or 0–6) or the name of the day.

SQL databases handle day-of-week queries differently:

1. MySQL

   SELECT DAYOFWEEK('2025-01-31') AS day_number,
          DAYNAME('2025-01-31') AS day_name;
   

   1. SQL Server sql SELECT DATEPART(dw, '2025-01-31') AS day_number, DATENAME(dw, '2025-01-31') AS day_name;

2. PostgreSQL

   SELECT EXTRACT(DOW FROM '2025-01-31'::date) AS day_number,
          TO_CHAR('2025-01-31'::date, 'Day') AS day_name;
   

   Key Considerations

   • Different databases may have different default first days of the week (Sunday or Monday).
   • Some systems use 1–7 numbering, while others use 0–6.
   • The ISO 8601 standard defines Monday as the first day of the week.

Using day of week functions, you can perform tasks like:

• Grouping data by weekday: Organizing records based on the day of the week (e.g., Monday, Tuesday) to analyze patterns, trends, or behaviors tied to specific days.
• Calculating business days: Determining the number of working days between two dates, excluding weekends and holidays, to calculate deadlines or project timelines.
• Scheduling recurring events: Planning events or tasks that occur on specific days of the week, such as weekly meetings or maintenance schedules.

Example: Sales Analysis

SELECT DAYNAME(order_date) AS weekday,
       SUM(total_amount) AS daily_sales
FROM orders
GROUP BY DAYNAME(order_date)
ORDER BY daily_sales DESC;

This query helps identify which day of the week has the highest sales, providing valuable insights for business decisions.

Beyond these examples, day-of-week functions can be used in more complex scenarios such as identifying weekends vs weekdays for workload balancing, adjusting pricing dynamically based on demand, optimizing staffing schedules based on historical trends, filtering time-series data for specific days, and analyzing seasonality in sales or user activity.

In SQL interviews, day-of-week functions are often used in combination with other date manipulation techniques. Common patterns include:

• Week-over-week comparisons: Analyzing changes in data over consecutive weeks.
• Business day calculations: Accounting for holidays and weekends when calculating deadlines.
• Holiday handling: Excluding specific holidays from business day calculations.

Example: Week-over-Week Comparison

WITH weekly_sales AS (
  SELECT 
    EXTRACT(WEEK FROM order_date) AS week_number,
    SUM(total_amount) AS weekly_sales
  FROM orders
  GROUP BY EXTRACT(WEEK FROM order_date)
)
SELECT 
  week_number,
  weekly_sales,
  LAG(weekly_sales) OVER (ORDER BY week_number) AS prev_week_sales
FROM weekly_sales
ORDER BY week_number;

Edge Cases and Gotchas

1. Date Format Issues: Ensure dates are properly formatted to avoid unexpected results.
2. NULL Values: Handle NULL dates carefully to prevent errors in calculations.
3. First Day of Week Variations: Be aware of database-specific settings for the first day of the week.

Performance Optimization Techniques

1. Indexing Date Columns: Improve query performance by indexing date fields.
2. Window Functions: Use window functions like LAG or LEAD for efficient comparisons between rows.
3. Avoid Correlated Subqueries: Opt for joins or window functions instead of correlated subqueries for better performance.

Example: Optimized Business Day Calculation

CREATE TABLE holidays (
  holiday_date DATE PRIMARY KEY
);

SELECT 
  order_date,
  CASE 
    WHEN DAYOFWEEK(order_date) IN (1, 7) OR order_date IN (SELECT holiday_date FROM holidays) THEN 0
    ELSE 1
  END AS is_business_day
FROM orders;

Interview Tips

• Clarify Assumptions: Always ask about the first day of the week and any specific date formats.
• Use Window Functions: Familiarize yourself with window functions for efficient data comparisons.
• Practice Handling NULLs: Ensure you can handle NULL dates effectively in your queries.

1. Find the Most Common Day for Orders

Given a table of orders with dates, determine which day of the week has the most orders.

Solution:

   SELECT DAYNAME(order_date) AS weekday, COUNT(*) AS order_count
   FROM orders
   GROUP BY DAYNAME(order_date)
   ORDER BY order_count DESC
   LIMIT 1;

2. Calculate Business Days Between Two Dates

Write a query to calculate the number of business days between two given dates, excluding weekends and holidays.

Solution:

   WITH business_days AS (
     SELECT 
       order_date,
       CASE 
         WHEN DAYOFWEEK(order_date) IN (1, 7) 
           OR order_date IN (SELECT holiday_date FROM holidays) THEN 0
         ELSE 1
       END AS is_business_day
     FROM orders
     WHERE order_date BETWEEN '2025-01-01' AND '2025-01-31'
   )
   SELECT SUM(is_business_day) AS total_business_days
   FROM business_days;

Mastering SQL day-of-week functions is not just about technical proficiency; it’s a stepping stone to advancing your career in data analysis and business intelligence. For a data analyst, learning SQL day-of-week functions is crucial because it enables the analysis of cyclic patterns, trends, and behaviors tied to specific days of the week. This skill helps answer critical questions about business cyclicality, user engagement, and operational efficiency, making it a fundamental tool for informed decision-making.

To grow with this skill:

1. Practice regularly: Apply these concepts to real-world datasets to solidify your understanding.
2. Explore advanced topics: Dive into time-series analysis and forecasting, which often rely on day-of-week patterns.
3. Learn complementary skills: Combine SQL with visualization tools like Tableau or Power BI to create impactful reports.
4. Contribute to open-source projects: Apply your skills to real-world problems and gain visibility in the data community.
5. Stay updated: Follow SQL blogs and attend webinars to keep up with the latest developments in date and time manipulation.

By honing your expertise in SQL date functions, including day-of-week operations, you’ll position yourself as a valuable asset in any data-driven organization. Although some might view day-of-week functions as basic, they are fundamental for creating more complex analyses. Understanding these functions is essential for building upon them to tackle advanced data challenges, such as analyzing seasonal trends or optimizing business operations based on day-of-week insights. This skill set is crucial for roles in data analysis, business intelligence, and even data science, opening doors to exciting career opportunities and projects that can significantly impact business decision-making.