SQL Tricks: Generate Calendar Table

Creating a "calendar table" or "date dimension" is a common task in SQL, especially for reporting, data warehousing, or when you need to perform calculations based on dates that might not exist in your actual data (e.g., finding days with no sales). While a full-fledged calendar table usually contains many attributes (day of week, week number, quarter, holiday flags, etc.), sometimes you just need a simple list of dates for a specific period, like the current month.

In this post, we'll explore how to dynamically generate a table containing all dates for the current month across different popular RDBMS dialects: MySQL, PostgreSQL, MS SQL Server, and Oracle. This approach avoids hardcoding dates and ensures your script always works for the current period.

Why generate a calendar table?
Before we dive into the code, let's briefly touch upon why this is useful:

• Filling Gaps: If your transaction data only records days with activity, a calendar table can help you identify days with no activity (e.g., zero sales).
• Time-Series Analysis: Essential for analyses that require a continuous timeline, even when data is sparse.
• Simplifying Joins: You can join your data to a calendar table to easily group by specific date parts or filter by continuous date ranges.
• Reporting: Providing a complete date range for reports, even if some dates have no associated data.

Let's look at the solutions for each database system.

MySQL

MySQL offers a few ways to generate series. We'll use a common table expression (CTE) combined with a recursive CTE or a numbers table approach to generate our dates.

WITH RECURSIVE dates AS (
    SELECT
        DATE_SUB(CURDATE(), INTERVAL DAYOFMONTH(CURDATE()) - 1 DAY) AS dt -- First day of current month
    UNION ALL
    SELECT
        DATE_ADD(dt, INTERVAL 1 DAY)
    FROM
        dates
    WHERE
        dt < LAST_DAY(CURDATE()) -- Last day of current month
)
SELECT
    dt AS calendar_date
FROM
    dates;

Explanation:

• WITH RECURSIVE dates AS (...): Defines a recursive CTE named dates.
• Anchor Member: SELECT DATE_SUB(CURDATE(), INTERVAL DAYOFMONTH(CURDATE()) - 1 DAY): This calculates the first day of the current month. CURDATE() gets the current date, DAYOFMONTH(CURDATE()) gets the day of the month (e.g., 15 for July 15th), and we subtract (day - 1) days to get to the 1st of the month.
• Recursive Member: SELECT DATE_ADD(dt, INTERVAL 1 DAY) FROM dates WHERE dt < LAST_DAY(CURDATE()): This part adds one day to the previous date (dt) until it reaches the last day of the current month, which is obtained using LAST_DAY(CURDATE()).

Try this solution with SQLize.online

PostgreSQL

PostgreSQL has a very convenient generate_series() function which is perfect for this task.

SELECT
    GENERATE_SERIES(
        DATE_TRUNC('month', CURRENT_DATE), -- First day of current month
        (DATE_TRUNC('month', CURRENT_DATE) + INTERVAL '1 month' - INTERVAL '1 day')::date, -- Last day of current month
        '1 day'
    )::date AS calendar_date;

Explanation:

• GENERATE_SERIES(start, stop, step): Generates a series of values.
• DATE_TRUNC('month', CURRENT_DATE): Truncates the current date to the beginning of the month, giving us the first day.
• (DATE_TRUNC('month', CURRENT_DATE) + INTERVAL '1 month' - INTERVAL '1 day'):📅 This calculates the last day of the current month. We go to the beginning of the next month (+ INTERVAL '1 month') and then subtract one day (- INTERVAL '1 day') to get the last day of the current month.
• '1 day': Specifies that each step in the series should be one day.
• :📅 Casts the resulting timestamp to a date type for a cleaner output.

Test tris code on SQLize.online

MS SQL Server

SQL Server offers multiple ways to generate sequences. Since SQL Server 2022, the GENERATE_SERIES function provides a straightforward method, similar to PostgreSQL. For earlier versions, or if you prefer, recursive CTEs are also a good option.

Using GENERATE_SERIES (SQL Server 2022+)

SELECT
    DATEADD(day, value, DATEADD(month, DATEDIFF(month, 0, GETDATE()), 0)) AS calendar_date
FROM
    GENERATE_SERIES(0, DAY(EOMONTH(GETDATE())) - 1);

Explanation:

• GENERATE_SERIES(start, stop, step): This function generates a sequence of numbers from start to stop with increments of step. By default, step is 1 if omitted.
• DATEADD(month, DATEDIFF(month, 0, GETDATE()), 0): This calculates the first day of the current month. This will be our base date.
• DAY(EOMONTH(GETDATE())) - 1: EOMONTH(GETDATE()) returns the last day of the current month. DAY() extracts the day number (e.g., 31 for July 31st). Subtracting 1 gives us the maximum number to generate for our series (from 0 to days_in_month - 1). For example, if a month has 31 days, we want to generate numbers from 0 to 30.
• DATEADD(day, value, ...): For each value generated by GENERATE_SERIES (which are 0, 1, 2, ... up to days_in_month - 1), we add that number of days to our first day of current month base date. This effectively generates each date of the month.

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Using Recursive CTE (Older SQL Server Versions or Alternative)

WITH Dates AS (
    SELECT
        DATEADD(month, DATEDIFF(month, 0, GETDATE()), 0) AS dt -- First day of current month
    UNION ALL
    SELECT
        DATEADD(day, 1, dt)
    FROM
        Dates
    WHERE
        DATEPART(month, DATEADD(day, 1, dt)) = DATEPART(month, GETDATE())
)
SELECT
    dt AS calendar_date
FROM
    Dates
OPTION (MAXRECURSION 366); -- Set max recursion limit, 366 covers leap years

Explanation:

• WITH Dates AS (...): Defines a recursive CTE named Dates.
• Anchor Member: SELECT DATEADD(month, DATEDIFF(month, 0, GETDATE()), 0) AS dt: This is a common SQL Server idiom to get the first day of the current month.
• DATEDIFF(month, 0, GETDATE()): Calculates the number of month boundaries crossed between 0 (which SQL Server treats as January 1, 1900) and GETDATE().
• DATEADD(month, ..., 0): Adds that number of months to 0, effectively landing on the first day of the current month.
• Recursive Member: SELECT DATEADD(day, 1, dt) FROM Dates WHERE DATEPART(month, DATEADD(day, 1, dt)) = DATEPART(month, GETDATE()): Adds one day to dt as long as the month of the next date is still the current month.
• OPTION (MAXRECURSION 366): Important for recursive CTEs in SQL Server. It sets the maximum number of times the recursive part can execute. 366 is a safe number to cover all possible days in a year (including leap years).

Try legacy SQL Server code

Oracle

Oracle provides a powerful CONNECT BY LEVEL clause, often used for generating sequences.

SELECT
    TRUNC(SYSDATE, 'MM') + LEVEL - 1 AS calendar_date
FROM
    dual
CONNECT BY
    TRUNC(SYSDATE, 'MM') + LEVEL - 1 <= LAST_DAY(SYSDATE);

Explanation:

• TRUNC(SYSDATE, 'MM'): Truncates the current date (SYSDATE) to the first day of the current month.
• LEVEL: A pseudo-column in hierarchical queries that returns the current level in the hierarchy (starting from 1).
• TRUNC(SYSDATE, 'MM') + LEVEL - 1: Generates successive dates starting from the first day of the month.
• When LEVEL is 1, it's first_day_of_month + 1 - 1 = first_day_of_month.
• When LEVEL is 2, it's first_day_of_month + 2 - 1 = first_day_of_month + 1 day.
• FROM dual: dual is a dummy table in Oracle, often used for selecting pseudo-columns or evaluating expressions.
• CONNECT BY TRUNC(SYSDATE, 'MM') + LEVEL - 1 <= LAST_DAY(SYSDATE): This clause acts as the loop condition. It continues generating rows as long as the generated date is less than or equal to the last day of the current month (LAST_DAY(SYSDATE)).

Oracle SQL playground online

Conclusion

As you can see, while the syntax differs across RDBMS, the core concept of generating a series of dates remains similar. With the addition of GENERATE_SERIES in SQL Server 2022, modern SQL versions are increasingly standardizing on easier ways to achieve this. Understanding these techniques is crucial for effective data manipulation and reporting in SQL. Choose the method that best suits your specific database environment and version.

I hope this was helpful! Feel free to leave a comment if you have any questions or alternative approaches.