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// 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.
/// Represents the behavior for null values when evaluating equality. Currently, its primary use
/// case is to define the behavior of joins for null values.
///
/// # Examples
///
/// The following table shows the expected equality behavior for `NullEquality`.
///
/// | A | B | NullEqualsNothing | NullEqualsNull |
/// |------|------|-------------------|----------------|
/// | NULL | NULL | false | true |
/// | NULL | 'b' | false | false |
/// | 'a' | NULL | false | false |
/// | 'a' | 'b' | false | false |
///
/// # Order
///
/// The order on this type represents the "restrictiveness" of the behavior. The more restrictive
/// a behavior is, the fewer elements are considered to be equal to null.
/// [NullEquality::NullEqualsNothing] represents the most restrictive behavior.
///
/// This mirrors the old order with `null_equals_null` booleans, as `false` indicated that
/// `null != null`.