Fix findCount()/exists() wrapping a raw CTE (WITH clause) for SQL Server

ebean-core/src/main/java/io/ebeaninternal/server/query/CQueryBuilder.java

@@ -321,6 +321,46 @@ static int lastTopLevelOrderBy(String sql) {
     return lastFound;
   }
 
+  /**
+   * Find the index of the top-level (non-nested) "select" keyword in sql. Used to detect if sql
+   * starts with a WITH clause (CTE) header - SQL Server does not support a WITH clause nested
+   * inside a subquery/derived table, so it must be hoisted in front of a wrapping SELECT
+   * (count/exists) rather than wrapped along with the rest of the query.
+   * <p>
+   * Returns 0 if there is no leading WITH clause (sql starts directly with SELECT), or -1 if no
+   * top-level SELECT is found at all.
+   */
+  static int topLevelSelectStart(String sql) {
+    int depth = 0;
+    int len = sql.length();
+    for (int i = 0; i < len; i++) {
+      char c = sql.charAt(i);
+      if (c == '(') {
+        depth++;
+      } else if (c == ')') {
+        depth--;
+      } else if (depth == 0 && sql.regionMatches(true, i, "select", 0, 6)
+        && (i == 0 || !Character.isLetterOrDigit(sql.charAt(i - 1)))
+        && (i + 6 == len || !Character.isLetterOrDigit(sql.charAt(i + 6)))) {
+        return i;
+      }
+    }
+    return -1;
+  }
+
+  /**
+   * Split off a leading WITH clause (CTE header) from sql, returning {@code {header, body}} so the
+   * header can be hoisted in front of a wrapping SELECT. Returns an empty header (unchanged sql as
+   * the body) when there is no leading WITH clause.
+   */
+  static String[] splitCteHeader(String sql) {
+    int pos = topLevelSelectStart(sql);
+    if (pos <= 0) {
+      return new String[]{"", sql};
+    }
+    return new String[]{sql.substring(0, pos), sql.substring(pos)};
+  }
+
   static String inlineSqlCommentLabel(String label, ProfileLocation profileLocation, boolean secondary, String simpleName) {
     if (label != null) {
       return secondary ? label : CQueryPlan.planLabelWithType(label, simpleName);
@@ -329,18 +369,22 @@ static String inlineSqlCommentLabel(String label, ProfileLocation profileLocatio
   }
 
   private String wrapSelectCount(String sql) {
-    sql = "select count(*) from ( " + sql + ")";
+    String[] parts = splitCteHeader(sql);
+    sql = parts[0] + "select count(*) from ( " + parts[1] + ")";
     if (selectCountWithAlias) {
       sql += " as c";
     }
     return sql;
   }
 
   static String wrapSelectExists(String sql, boolean existsWithCaseWhen, String existsFromClause) {
+    String[] parts = splitCteHeader(sql);
+    String header = parts[0];
+    String body = parts[1];
     if (existsWithCaseWhen) {
-      return "select case when exists(" + sql + ") then 1 else 0 end" + existsFromClause;
+      return header + "select case when exists(" + body + ") then 1 else 0 end" + existsFromClause;
     }
-    return "select exists(" + sql + ")";
+    return header + "select exists(" + body + ")";
   }
 
   /**

ebean-core/src/test/java/io/ebeaninternal/server/query/CQueryBuilderTest.java

@@ -117,6 +117,63 @@ void countWrap_simpleOrderByIsStripped() {
     assertThat(countSql).isEqualTo("select count(*) from ( select t0.id from ad t0) as c");
   }
 
+  @Test
+  void topLevelSelectStart_noLeadingCte_returnsZero() {
+    String sql = "select 1 from o_order t0 where t0.id > ?";
+    assertThat(CQueryBuilder.topLevelSelectStart(sql)).isEqualTo(0);
+  }
+
+  @Test
+  void topLevelSelectStart_leadingCte_findsOuterSelect() {
+    // The only depth-0 "select" is the outer query - the CTE body's "select" is nested in parens.
+    String sql = "with order_totals as (" +
+      "  select o.id as order_id," +
+      "         sum(d.order_qty * d.unit_price) as total_amount" +
+      "  from o_order o" +
+      "  join o_order_detail d on d.order_id = o.id" +
+      "  group by o.id" +
+      ")" +
+      " select order_id, total_amount" +
+      " from order_totals" +
+      " where total_amount > ?";
+
+    int pos = CQueryBuilder.topLevelSelectStart(sql);
+    assertThat(sql.substring(pos)).startsWith("select order_id, total_amount");
+  }
+
+  /**
+   * SQL Server does not support a WITH clause (CTE) nested inside a subquery/derived table - see
+   * https://github.com/ebean-orm/ebean/issues/3848 (findCount() wraps raw sql in "select count(*)
+   * from ( ... )" which breaks when the raw sql is a CTE). The CTE header must be hoisted in front
+   * of the wrapping SELECT.
+   */
+  @Test
+  void splitCteHeader_hoistsLeadingWithClause() {
+    String sql = "with order_totals as (" +
+      "  select o.id as order_id," +
+      "         sum(d.order_qty * d.unit_price) as total_amount" +
+      "  from o_order o" +
+      "  join o_order_detail d on d.order_id = o.id" +
+      "  group by o.id" +
+      ")" +
+      " select order_id, total_amount" +
+      " from order_totals" +
+      " where total_amount > ?";
+
+    String[] parts = CQueryBuilder.splitCteHeader(sql);
+    assertThat(parts[0] + parts[1]).isEqualTo(sql);
+    assertThat(parts[0]).startsWith("with order_totals as (").endsWith(") ");
+    assertThat(parts[1]).isEqualTo("select order_id, total_amount from order_totals where total_amount > ?");
+  }
+
+  @Test
+  void splitCteHeader_noCte_returnsEmptyHeader() {
+    String sql = "select 1 from o_order t0 where t0.id > ?";
+    String[] parts = CQueryBuilder.splitCteHeader(sql);
+    assertThat(parts[0]).isEmpty();
+    assertThat(parts[1]).isEqualTo(sql);
+  }
+
   @Test
   void wrapSelectExists_default_usesScalarExists() {
     String sql = CQueryBuilder.wrapSelectExists("select 1 from o_order t0 where t0.id > ?", false, "");