fix: alias wrapped ClickHouse aggregate text columns to keep result-set label

Author: luciano-fiandesio

dhis-2/dhis-services/dhis-service-analytics/src/main/java/org/hisp/dhis/analytics/event/data/AbstractJdbcEventAnalyticsManager.java

@@ -703,14 +703,24 @@ protected ColumnAndAlias getColumnAndAlias(
         && hasOrderByClauseForQueryItem(queryItem, params)) {
       return getColumnAndAliasWithNullIfFunction(queryItem);
     } else if (queryItem.isText() && isAggregated) {
-      // ClickHouse stores an empty string value as '' rather than NULL, so a raw GROUP BY would
-      // produce a '' bucket and return '' instead of NULL. nullIfEmpty normalises '' to
+      // ClickHouse stores an absent string value as '' rather than NULL, so a raw GROUP BY would
+      // produce a spurious '' bucket and return '' instead of NULL. nullIfEmpty normalises '' to
       // NULL (no-op on Postgres/Doris). The same expression is used for SELECT and GROUP BY so the
       // bucket key and the displayed value stay aligned.
-      String column = sqlBuilder.nullIfEmpty(getColumn(queryItem));
+      String rawColumn = getColumn(queryItem);
+      String column = sqlBuilder.nullIfEmpty(rawColumn);
+
+      if (column.equals(rawColumn)) {
+        // No normalisation applied (Postgres/Doris): keep the original rendering unchanged.
+        return getColumnAndAlias(queryItem, isGroupByClause, "");
+      }
+
+      // ClickHouse: the nullif wrapper drops the implicit column label, so alias it with the item
+      // name the row builder looks up.
       return isGroupByClause
           ? ColumnAndAlias.ofColumn(column)
-          : ColumnAndAlias.ofColumnAndAlias(column, getAlias(queryItem).orElse(""));
+          : ColumnAndAlias.ofColumnAndAlias(
+              column, getAlias(queryItem).orElse(queryItem.getItemName()));
     } else {
       return getColumnAndAlias(queryItem, isGroupByClause, "");
     }

dhis-2/dhis-services/dhis-service-analytics/src/test/java/org/hisp/dhis/analytics/event/data/EventAnalyticsManagerTest.java

@@ -1048,6 +1048,8 @@ void verifyAggregatedTextDimensionWrapsEmptyStringAsNullForClickHouse() {
     String generatedSql = sql.getValue().toLowerCase();
     // SELECT and GROUP BY must both reference the nullif-wrapped column so '' folds into NULL.
     assertThat(countMatches(generatedSql, "nullif(ax.\"fwiaetyvegk\", '')"), is(2));
+    // The SELECT column must keep an alias matching the item name so the row builder can read it.
+    assertThat(generatedSql, containsString("nullif(ax.\"fwiaetyvegk\", '') as \"fwiaetyvegk\""));
   }
 
   @Test