Which function retrieves a specific character from a string?

The SUBSTR function is designed to retrieve a specific substring from a string based on the starting position and the length of the substring to be extracted. It allows users to specify which part of the string they want to obtain by providing the starting index and the desired length of the substring. For example, if you have a string "Hello World" and you use the SUBSTR function to extract starting at position 1 and a length of 5, it will return "Hello". This functionality makes it the correct choice for retrieving specific characters or parts of a character string.

Understanding the roles of the other functions clarifies why they do not fit the question. The LENGTH function is used to determine the number of characters in a string, which provides information about the string's size rather than extracting a specific character. The TRIM function is focused on removing leading and trailing spaces from a string, without retrieving specific characters. Finally, the CONCAT function is used to combine two or more strings together into one, rather than extracting any part of a string. Thus, the SUBSTR function stands out as the appropriate choice for the task of retrieving specific characters from a string.