Compare dates in Oracle SQL

When comparing dates in Oracle SQL, be sure to make sure the date is formatted correctly. By default, Oracle interprets dates as numbers, which can result in errors if handled improperly.

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

Use TO_DATE()

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

select employee_id
  from employee
 where employee_date_hired > to_date('31-12-1995','DD-MM-YYYY')

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 literal is another way to specify dates in Oracle SQL. They are written in 'YYY-MM-DD' format and do not contain time elements. For example:

select employee_id
  from employee
 where employee_date_hired > date '1995-12-31'

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

Processing time and locale settings

Oracle dates contain time components, so if you need to make precise comparisons, be sure to consider the time element. Additionally, date formats may vary depending on NLS_DATE_FORMAT and NLS_DATE_LANGUAGE settings. If you need to ensure consistent date processing across regions, use the YYYY-MM-DD format and set NLS_DATE_FORMAT and NLS_DATE_LANGUAGE accordingly.

By employee count

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

select employee_id, count(*)
  from employee
 where employee_date_hired > date '1995-12-31'
 group by employee_id

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