Which of the following is used to indicate a single-line comment in structured query language (SQL)?

The double hyphen (--) is defined by the ANSI SQL standard as the single-line comment syntax; every character after -- on that line is ignored by the SQL engine at parse time. This syntax is also commonly exploited in SQL injection attacks to truncate and neutralize the remainder of a query.

The double pipe (||) is the SQL string concatenation operator used to join two string expressions and has no role as a comment delimiter.

The percent sign (%) serves as a wildcard in SQL LIKE predicates; double percent (%%) has no defined meaning as a comment syntax in standard SQL.

Two consecutive single quotes ('') represent an escaped single-quote character or an empty string literal in SQL and are not treated as a comment delimiter.