fix: 考试分析-最难题目SQL在不选班级时缺少空格导致语法错误，点分析报错

src/main/java/com/sztzjy/linkCommerce/service/ExamService.java

@@ -433,14 +433,7 @@ public class ExamService {
                 bucket("良好", scores, total.multiply(new BigDecimal("0.80")), total.multiply(new BigDecimal("0.90"))),
                 bucket("及格", scores, pass, total.multiply(new BigDecimal("0.80"))),
                 bucket("不及格", scores, BigDecimal.ZERO, pass));
-        String hardestSql = "select p.question_id id,p.stem,p.question_score, count(distinct t.id) student_count, " +
+        String hardestSql = analysisHardestSql(StringUtils.isNotBlank(classId));
-                "sum(case when coalesce(a.awarded_score,0) < p.question_score then 1 else 0 end) wrong_count, " +
-                "coalesce(sum(coalesce(a.awarded_score,0)),0) earned_score " +
-                "from exam_assessment_question p join exam_attempt t on t.exam_id=p.exam_id and t.attempt_status='SUBMITTED' " +
-                "left join exam_attempt_answer a on a.attempt_id=t.id and a.question_id=p.question_id " +
-                "where p.exam_id=?" + (StringUtils.isBlank(classId) ? "" : " and t.teaching_class_id=? ") +
-                "group by p.question_id,p.stem,p.question_score " +
-                "order by (case when count(distinct t.id)=0 then 100 else coalesce(sum(coalesce(a.awarded_score,0)),0) * 100.0 / (count(distinct t.id) * p.question_score) end) asc, wrong_count desc limit 3";
         List<Map<String, Object>> hardest = jdbc.query(hardestSql,
                 (rs, n) -> {
                     int students = rs.getInt("student_count");
@@ -754,6 +747,17 @@ public class ExamService {
         if (!expected.equals(actual)) throw new IllegalArgumentException(message);
     }
 
+    static String analysisHardestSql(boolean withClassFilter) {
+        return "select p.question_id id,p.stem,p.question_score, count(distinct t.id) student_count, " +
+                "sum(case when coalesce(a.awarded_score,0) < p.question_score then 1 else 0 end) wrong_count, " +
+                "coalesce(sum(coalesce(a.awarded_score,0)),0) earned_score " +
+                "from exam_assessment_question p join exam_attempt t on t.exam_id=p.exam_id and t.attempt_status='SUBMITTED' " +
+                "left join exam_attempt_answer a on a.attempt_id=t.id and a.question_id=p.question_id " +
+                "where p.exam_id=? " + (withClassFilter ? "and t.teaching_class_id=? " : "") +
+                "group by p.question_id,p.stem,p.question_score " +
+                "order by (case when count(distinct t.id)=0 then 100 else coalesce(sum(coalesce(a.awarded_score,0)),0) * 100.0 / (count(distinct t.id) * p.question_score) end) asc, wrong_count desc limit 3";
+    }
+
     static void validateDelete(String status, Object exportedAt) {
         if ("DRAFT".equals(status)) return;
         requireState(status, "ENDED", "只有未发布或已结束的考试可以删除");

src/test/java/com/sztzjy/linkCommerce/service/ExamServiceTest.java

@@ -58,6 +58,15 @@ class ExamServiceTest {
         }
     }
 
+    @Test
+    void analysisHardestSqlKeepsParameterAndGroupBySeparated() {
+        for (String sql : Arrays.asList(ExamService.analysisHardestSql(false), ExamService.analysisHardestSql(true))) {
+            assertFalse(sql.contains("?group"));
+            assertTrue(sql.contains("? group by p.question_id"));
+        }
+        assertTrue(ExamService.analysisHardestSql(true).contains("? and t.teaching_class_id=? group by"));
+    }
+
     @Test
     void studentAnswerFilesStillRejectExcel() {
         IllegalArgumentException error = assertThrows(IllegalArgumentException.class,