Why Is MySQL Date Comparison with date_format Yielding Unexpected Results?

Comparing MySQL Dates with date_format

Despite attempting multiple date comparison methods, a user encountered an unexpected result. They sought to compare dates formatted as '%d-%m-%Y' but included an erroneous record in the result set.

The issue stems from comparing strings instead of dates. The date_format function converts dates into strings, making the comparison susceptible to lexicographical ordering. For instance, '28-10-2012' is alphabetically larger than '02-11-2012', hence its inclusion in the result set despite being chronologically earlier.

To resolve this, the comparison should be performed on dates instead of strings. The date function can be used to extract the date component from a DATETIME or DATE field. By comparing the extracted dates, chronological order is ensured.

For example:

<code class="mysql">select date_format(date(starttime),'%d-%m-%Y') from data
where date(starttime) >= date '2012-11-02';</code>

In this query, date(starttime) extracts the date from the starttime field, which is then compared to the specified date in YYYY-MM-DD format. The date_format function is used only for formatting the result.

If starttime is a DATETIME field, consider using:

<code class="mysql">select date_format(date(starttime),'%d-%m-%Y') from data
where starttime >= '2012-11-02 00:00:00';</code>

This query avoids repeated date conversion as the comparison is performed directly on the DATETIME field.

Remember, the ISO-8601 standard recommends using 'YYYY-MM-DD' as the date format, but the provided code adheres to the user's specified '%d-%m-%Y' format.

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