How to Correctly Compare Dates in Oracle SQL?

Compare Dates in Oracle SQL

When comparing dates in Oracle SQL, it is important to ensure that the date format is correct. By default, Oracle interprets dates as numbers, which can cause errors if not handled properly.

A common problem is using strings to represent dates. Strings (e.g. '31-DEC-95') are not recognized as valid dates and will trigger an "invalid identifier" error. To resolve this issue, use a date literal or the TO_DATE() function to convert the string to a date.

Use TO_DATE()

TO_DATE() function allows you to specify the format of a date string and convert it to a date data type. For example:

<code class="language-sql">select employee_id
  from employee
 where employee_date_hired > to_date('31-12-1995','DD-MM-YYYY')</code>

This query converts the string '31-12-1995' to a date using the 'DD-MM-YYYY' format and compares it with the employee_date_hired column.

Use date text

Date literals are another way to specify dates in Oracle SQL. They are written in 'YYYY-MM-DD' format and do not contain a time element. For example:

<code class="language-sql">select employee_id
  from employee
 where employee_date_hired > date '1995-12-31'</code>

This query compares the employee_date_hired column to the date literal '1995-12-31' (representing December 31, 1995).

Processing time and locale

Oracle dates contain a time component, so be sure to consider the time element if you need to make an exact comparison. Additionally, the date format may vary depending on the NLS_DATE_FORMAT and NLS_DATE_LANGUAGE settings. If you need to ensure consistent date handling across locales, use the YYYY-MM-DD format and set NLS_DATE_FORMAT and NLS_DATE_LANGUAGE accordingly.

Count by employees

To count the number of employees hired after a specific date and group the results by employee ID, you can use the following query:

<code class="language-sql">select employee_id, count(*)
  from employee
 where employee_date_hired > date '1995-12-31'
 group by employee_id</code>

This query provides the number of employees hired after December 31, 1995, grouped by their employee ID.

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