Getting Error – ORA-01858: a non-numeric character was found where a numeric was expected

The error you are getting is either because you are doing TO_DATE on a column that’s already a date, and you’re using a format mask that is different to your nls_date_format parameter[1] or because the event_occurrence column contains data that isn’t a number.

You need to a) correct your query so that it’s not using TO_DATE on the date column, and b) correct your data, if event_occurrence is supposed to be just numbers.

And fix the datatype of that column to make sure you can only store numbers.

[1] What Oracle does when you do: TO_DATE(date_column, non_default_format_mask) is:
TO_DATE(TO_CHAR(date_column, nls_date_format), non_default_format_mask)

Generally, the default nls_date_format parameter is set to dd-MON-yy, so in your query, what is likely to be happening is your date column is converted to a string in the format dd-MON-yy, and you’re then turning it back to a date using the format MMDD. The string is not in this format, so you get an error.

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