Which SQL function is used to extract a substring from a string variable?

• A. CONCAT
• B. REPLACE
• C. TRIM
• D. SUBSTR

Answer: (D)

Substring extraction from a string uses a function that takes the string, a starting position, and an optional length, returning just that portion. The function to use for this is SUBSTR, which returns the characters starting at a given index and, if provided, the number of characters to include. For example, SUBSTR('Data', 2, 3) yields 'ata'. This makes SUBSTR the best choice for obtaining a substring. Other string functions do different things: CONCAT joins two strings, REPLACE substitutes a specified substring with another value, and TRIM removes leading and trailing whitespace (or other characters). Some SQL dialects use SUBSTRING instead of SUBSTR, but the concept is the same: you specify the string, a start position, and an optional length to extract a substring.