content: /*
   * Licensed to the Apache Software Foundation (ASF) under one or more
   * contributor license agreements.  See the NOTICE file distributed with
   * this work for additional information regarding copyright ownership.
   * The ASF licenses this file to you under the Apache License, Version 2.0
   * (the "License"); you may not use this file except in compliance with
   * the License.  You may obtain a copy of the License at
   *
   * http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  package org.apache.calcite.sql;
  
  import org.apache.calcite.rel.core.AggregateCall;
  import org.apache.calcite.rel.type.RelDataType;
  import org.apache.calcite.rex.RexBuilder;
  import org.apache.calcite.rex.RexNode;
  
  /** Aggregate function that knows how to convert itself to a scalar value
   * when applied to a single row. */
  public interface SqlSingletonAggFunction {
    /** Generates an expression for the value of the aggregate function when
     * applied to a single row.
     *
     * <p>For example, if there is one row:
     * <ul>
     *   <li>{@code SUM(x)} is {@code x}
     *   <li>{@code MIN(x)} is {@code x}
     *   <li>{@code MAX(x)} is {@code x}
     *   <li>{@code COUNT(x)} is {@code CASE WHEN x IS NOT NULL THEN 1 ELSE 0 END 1}
     *   which can be simplified to {@code 1} if {@code x} is never null
     *   <li>{@code COUNT(*)} is 1
     *   <li>{@code GROUPING(deptno)} if 0 if {@code deptno} is being grouped,
     *       1 otherwise
     * </ul>
     *
     * @param rexBuilder Rex builder
     * @param inputRowType Input row type
     * @param aggregateCall Aggregate call
     *
     * @return Expression for single row
     */
    RexNode singleton(RexBuilder rexBuilder, RelDataType inputRowType,
        AggregateCall aggregateCall);
  }
  