In Oracle, you can use the `INSTR` function to check if a character exists within a string. The `INSTR` function takes two arguments: the substring you want to find, and the string you want to search within. If the substring is found within the string, the `INSTR` function will return the position of the first occurrence of the substring. If the substring is not found, the `INSTR` function will return 0.
For example, the following query would check if the character ‘a’ exists within the string ‘Hello, world!’: