Don't compare datetimes with date ranges in MySQL

When selecting records in a date range, take care not to do it like this:

start_date = Date.parse('2007-05-01')
end_date = Date.parse('2007-05-31')
LogItem.where(:created_at => start_date .. end_date)

The problem is that created_at is a datetime (or Time in Ruby), while start_date and end_date are simple dates. In order to make sense of your query, MySQL will cast your dates to datetimes where the time component is 00:00:00. Because of this the query above will lose records created from 2007-05-31 00:00:01 to 2007-05-31 23:59:59.

Details

The Ruby code above triggers the following query:

SELECT * FROM log_items WHERE created_at BETWEEN '2007-05-01' AND '2007-05-31'

MySQL sees that you are trying to compare a datetime with a date and silently casts your query to:

SELECT * FROM log_items WHERE created_at BETWEEN '2007-05-01 00:00:00' AND '2007-05-31 00:00:00'

Note how you just lost all records from 2007-05-31 00:00:01 to 2007-05-31 23:59:59.

Solutions

One solution is to cast the datetime to a date in MySQL:

LogItem.where('DATE(log_items.created_at) BETWEEN ? AND ?', start_date, end_date)

Another solution is to be more explicit about what you want to select:

LogItem.where(:created_at => start_date.beginning_of_day..end_date.end_of_day)
Henning Koch about 9 years ago
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