How to Fill Missing Dates in SQL Query Results with Zero Counts?

Padding Empty Dates in SQL Results

When querying MySQL to export dates and counts from a table, gaps in the date range can leave some days unaccounted for. To fill these gaps with zero-count entries, a straightforward approach is to utilize a server-side stored procedure.

Server-Side Padding with Stored Procedure

The following stored procedure creates a temporary table containing all possible dates within a specified range. This table is then 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;
end procedure

This procedure takes two dates as input and generates a temporary table foo with all potential dates between them. It then performs a left join with the original query to include any missing dates and return a complete dataset.

Client-Side Check (Optional)

Additionally, a simple check can be implemented on the client side (e.g., Perl) to handle cases where the current date is not equal to the previous date plus one day. In such cases, additional strings with zero counts can be added to the output.

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