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to_number [Oracle SQL]

to_number(expr)
to_number(expr,format)
to_number(expr,format,'nls-param')
The behaviour of to_number can be influenced by setting the NLS_NUMERIC_CHARACTERS, NLS_CURRENCY, NLS_DUAL_CURRENCY and NLS_ISO_CURRENCY NLS parameters.
If expr cannot be converted into a number, an ORA-06502 (or value_error) is thrown. This makes it possible to check if a string is numeric.

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See also safe_to_number: which is a function that converts text safely to numbers if they are convertable and returns null otherwise. That is, it prevents the ORA-01722: invalid number error which is thrown if to_number is passed a string that doesn't represent a number.