How to Fill Missing Dates in SQL Query Results?

Pad Missing Dates in SQL Results

When querying a database for a date range, it's common to encounter gaps in the data. To address this, there are several straightforward approaches to pad the results with empty dates and corresponding counts.

On the MySQL Side

One option is to create a stored procedure that generates a temporary table with all the possible dates within the specified range. This table can then be left joined with the query results to fill in the missing dates.

CREATE PROCEDURE sp1(d1 DATE, d2 DATE)
DECLARE d DATETIME;

CREATE TEMPORARY TABLE foo (d DATE NOT NULL);

SET d = d1
WHILE d <= d2 DO
  INSERT INTO foo (d) VALUES (d)
  SET d = DATE_ADD(d, INTERVAL 1 DAY)
END WHILE

SELECT foo.d, COUNT(date)
FROM foo LEFT JOIN table ON foo.d = table.date
GROUP BY foo.d ORDER BY foo.d ASC;

DROP TEMPORARY TABLE foo;

On the Perl Side

If Perl modules like Date::y are available, you can iterate over a date range and check for empty dates in the query results. If found, add a zero-count entry for that date.

use Date::y;

# Get the date range
my $d1 = '2008-08-05';
my $d2 = '2008-08-07';

# Iterate over the date range
my $date = Date::y->date($d1);
while ($date <= Date::y->date($d2)) {
  # Check if the date exists in the query results
  if (! exists $results{$date}) {
    # Add the date with a zero count
    $results{$date} = 0;
  }
  $date = $date->next;
}

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