Below are two Oracle Database functions that convert uppercase characters to their lowercase equivalent.

The LOWER() Function

The LOWER() function returns its argument with all letters in lowercase.

Example:

SELECT LOWER('This is a COVERT OPERATION') 
FROM DUAL;

Result:

this is a covert operation

Any lowercase letters remain in lowercase, while uppercase characters are converted to their lowercase equivalent.

The NLS_LOWER() Function

The NLS_LOWER() function is similar to the LOWER() function, except that it accepts a second argument that allows you to specify the collation. The collation handles special linguistic requirements for case conversions. 

The second argument is optional, so we can use this function to do exactly the same as the previous example:

SELECT NLS_LOWER('This is a COVERT OPERATION') 
FROM DUAL;

Result:

this is a covert operation

However, we can also call the function with a second argument to specify the collation.

Example:

SELECT 
    NLS_LOWER('BALIQ') AS r1,
    NLS_LOWER('BALIQ', 'NLS_SORT = XAZERBAIJANI') AS r2
FROM DUAL;

Result:

      R1       R2 
________ ________ 
baliq    balıq   

Notice that the second column uses a dotless lowercase i character, which adheres to the Azerbaijani writing system.