How Can I Populate Missing Dates in MySQL Queries for Complete Date Ranges?

Populating Data Gaps for Date Ranges in MySQL

One may encounter scenarios where it becomes necessary to retrieve data between two specific dates, even if there are no entries for certain dates within that range. To address this challenge, a popular approach is to create "calendar tables."

Consider the following table structure:

CREATE TABLE data (
  date DATE,
  value INT
);

INSERT INTO data VALUES
  ('2009-06-25', 75),
  ('2009-07-01', 100),
  ('2009-07-02', 120);

To retrieve all data between '2009-06-25' and '2009-07-01', the following query can be used:

SELECT date, value
FROM data
WHERE date BETWEEN '2009-06-25' AND '2009-07-01';

However, this query will exclude any dates within the specified range that have no data associated with them. To address this, a calendar table is employed.

To create a calendar table for the given date range, the following steps can be taken:

CREATE TABLE cal (
  date DATE
);

INSERT INTO cal (date)
SELECT DATE_ADD('2009-06-25', INTERVAL i * 10 + j DAY)
FROM ints i CROSS JOIN ints j
WHERE DATE_ADD('2009-06-25', INTERVAL i * 10 + j DAY) BETWEEN '2009-06-25' AND '2009-07-01';

The cal table will contain all dates between '2009-06-25' and '2009-07-01', but without any associated data. To retrieve the desired data along with the gaps, a left join can be used:

SELECT cal.date, data.value
FROM cal
LEFT JOIN data ON cal.date = data.date
WHERE cal.date BETWEEN '2009-06-25' AND '2009-07-01';

This query will return all dates within the specified range, including those without data, which will be represented with NULL values for the value column.

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