How Can I Extract Data with Missing Dates from a MySQL Table Using a Calendar Table?

Extracting Data with Missing Dates Using Calendar Tables

Introduction:

The task at hand involves retrieving data from a MySQL table that includes date-associated values. The goal is to select all data that falls within a specified date range, including instances where data is absent for certain dates.

Query:

The provided query, SELECT data FROM tbl WHERE date BETWEEN date1 AND date2, effectively selects data within a defined range. However, it fails to account for empty dates within that interval.

Solution:

To address this issue, the concept of "calendar tables" can be employed. These tables provide a structured way of representing dates, allowing for easy manipulation and inclusion of missing dates.

Calendar Table Implementation:

The following MySQL code can be used to create a calendar table:

CREATE TABLE ints (i INTEGER);
INSERT INTO ints VALUES (0), (1), (2), (3), (4), (5), (6), (7), (8), (9);

To generate a calendar table that spans the specified date range:

SELECT '2009-06-25' + INTERVAL a.i * 10 + b.i DAY as date
FROM ints a JOIN ints b
ORDER BY a.i * 10 + b.i

This query will generate a list of dates within the range '2009-06-25' and '2009-07-01'.

Combining Calendar and Data Table:

By left joining the calendar table ('cal') with the original table ('tbl') on the 'date' field, we can obtain the desired results:

SELECT cal.date, tbl.data
FROM (
    SELECT '2009-06-25' + INTERVAL a.i * 10 + b.i DAY as date
    FROM ints a JOIN ints b
    ORDER BY a.i * 10 + b.i
) cal LEFT JOIN tbl ON cal.date = tbl.date
WHERE cal.date BETWEEN '2009-06-25' AND '2009-07-01';

This query will return all dates within the specified range, including those with empty data values (which will be displayed as NULL).

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