{keyword}') Union - All Select Null,null,null,null,null-- Dyyf

: This is likely a "fingerprint" or a random string used by automated security scanners (like Burp Suite or SQLmap) to identify which specific payload successfully triggered a response. Why you are seeing this

: This is the SQL comment syntax. It tells the database to ignore everything that follows it, which prevents the remaining "real" code from causing a syntax error. {KEYWORD}') UNION ALL SELECT NULL,NULL,NULL,NULL,NULL-- DyYf

: The attacker is trying to determine how many columns the original database table has. By providing five NULL values, they are testing if the original query also returns five columns (the number of columns must match for a UNION to work). : This is likely a "fingerprint" or a