How to Fill Date Gaps in MySQL Queries?

Filling Date Gaps in MySQL

When working with date-based queries in MySQL, you may encounter situations where there are gaps in the data for specific dates. This can be problematic if you want to present a comprehensive view of the data or perform calculations that rely on having a complete set of dates.

Query with Date Gaps:

To demonstrate the issue, consider the following query:

SELECT DATE(posted_at) AS date,
    COUNT(*) AS total,
    SUM(attitude = 'positive') AS positive,
    SUM(attitude = 'neutral') AS neutral,
    SUM(attitude = 'negative') AS negative
    FROM `messages`
    WHERE (`messages`.brand_id = 1)
    AND (`messages`.`spam` = 0
    AND `messages`.`duplicate` = 0
    AND `messages`.`ignore` = 0)
    GROUP BY date ORDER BY date

This query returns the results grouped by date, showing the total count and attitude sums for each date. However, if there are any dates with no data, they will not be included in the results, causing gaps in the dataset.

Filling the Gaps:

To fill these gaps with zeros, we can utilize a helper table to generate a complete set of dates between a start and end date. The helper table can be created using the following query:

SELECT date AS dt FROM (
    SELECT CURDATE() AS date
    UNION ALL
    SELECT date + INTERVAL 1 DAY FROM dates
    FROM dates
    WHERE date < DATE_ADD(CURDATE(), INTERVAL 1 YEAR)
) dates

This query recursively adds one day to the current date until it reaches one year into the future.

Query with Helper Table:

Now, we can join the helper table with our main query using a LEFT JOIN to fill the date gaps:

SELECT  d.dt AS date,
        COUNT(*) AS total,
        SUM(attitude = 'positive') AS positive,
        SUM(attitude = 'neutral') AS neutral,
        SUM(attitude = 'negative') AS negative
FROM    dates d
LEFT JOIN
        messages m
ON      m.posted_at >= d.dt
        AND m.posted_at < d.dt + INTERVAL 1 DAYS
        AND spam = 0
        AND duplicate = 0
        AND ignore = 0
GROUP BY
        d.dt
ORDER BY
        d.dt

This query will return a complete set of dates, including those with no data, and fill the gaps with zeros.

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