The default "&" in SQL * Plus indicates the substitution variable. That is to say, as long as the symbol appears in the command, SQL * Plus requires you to enter the substitution value. This means that you cannot input a string containing this symbol into the database or assign it to a variable. For example, the string "SQL & plus" system will understand it as a string with the "SQL" header, it prompts you to enter the value of the substitution variable plus. If you enter ABC, the final string is converted to "sqlabc ".
Set define off disables this function. "&" is used as a common character. In the preceding example, the final character is "SQL & plus"
Set define off disable the substitution Variable Function
Set define on enable the variable substitution function
Set define * indicates the default substitution variable identifier as "*" (or other characters)
