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DBMS_PIPE.RECEIVE_MESSAGE and DBMS_PIPE.RECEIVE_MESSAGE(chr(99)||chr(99)||chr(99), 15) are two functions in the Oracle database management system (DBMS) that are used to receive messages from a pipe. A pipe is a unidirectional communications channel used for inter-process communication.

Despite having similar names, these two functions have some key differences. Let's take a closer look at each of them and compare them.

DBMS_PIPE.RECEIVE_MESSAGE is a basic function that is used to receive one or more messages from a pipe. It takes in two parameters - pipe name and timeout. The pipe name specifies the pipe from which the messages are to be received, and the timeout specifies the maximum amount of time (in seconds) to wait for a message. If no message is received within the specified timeout, the function returns null.

On the other hand, DBMS_PIPE.RECEIVE_MESSAGE(chr(99)||chr(99)||chr(99), 15) is a more advanced version of the basic function. It also takes in two parameters, but the first parameter is an expression that evaluates to the pipe name. Here, the expression is chr(99)||chr(99)||chr(99), which evaluates to 'ccc'. This means that the pipe name is 'ccc', and the remaining functionality is the same as the basic function.

So, what's the difference between using the basic function and the advanced version with the expression? Well, using the expression gives you the flexibility to dynamically change the pipe name depending on some condition or variable. This can be useful if different parts of your code need to communicate with different pipes at different times.

Another thing to note is that the expression needs to evaluate to a valid pipe name, otherwise an error will be thrown. So, it's important to ensure that the expression is properly constructed and evaluated.

In summary, DBMS_PIPE.RECEIVE_MESSAGE and DBMS_PIPE.RECEIVE_MESSAGE(chr(99)||chr(99)||chr(99), 15) both serve the purpose of receiving messages from a pipe, but the latter provides more flexibility in terms of dynamically changing the pipe name. As always, when working with any DBMS function, it's important to understand their differences and use them appropriately.