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;
  
  /**
   * SqlNullSemantics defines the possible comparison rules for values which might
   * be null. In SQL (and internal plans used to process SQL) different rules are
   * used depending on the context.
   */
  public enum SqlNullSemantics {
    /**
     * Predicate semantics: e.g. in the expression (WHERE X=5), if X is NULL,
     * the comparison result is unknown, and so a filter used to evaluate the
     * WHERE clause rejects the row.
     */
    NULL_MATCHES_NOTHING,
  
    /**
     * GROUP BY key semantics: e.g. in the expression (GROUP BY A,B), the key
     * (null,5) is treated as equal to another key (null,5).
     */
    NULL_MATCHES_NULL,
  
    /**
     * Wildcard semantics: logically, this is not present in any SQL construct.
     * However, it is required internally, for example to rewrite NOT IN to NOT
     * EXISTS; when we negate a predicate, we invert the null semantics, so
     * NULL_MATCHES_NOTHING must become NULL_MATCHES_ANYTHING.
     */
    NULL_MATCHES_ANYTHING
  }
  