Oracle “Invalid Number” caused by to_char function

Question

I have the following SQL query which is looking for duplicate values in a table (two values being null must be classed as a duplicate, hence the use of nvl):

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Answer 1

To_Char expects to receive a number as its argument by default. If it receives a character string then it will try to implictly convert the string to a number, and that will raise this error message if the string cannot be converted.

If the column is of a character type, do not try to convert it to a string.