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.
   */
  
  /**
   * Provides a SQL parser and object model.
   *
   * <p>This package, and the dependent <code>org.apache.calcite.sql.parser</code>
   * package, are independent of the other Calcite packages, so may be used
   * standalone.
   *
   * <h2>Parser</h2>
   *
   * <p>{@link org.apache.calcite.sql.parser.SqlParser} parses a SQL string to a
   *     parse tree. It only performs the most basic syntactic validation.
   *
   * <h2>Object model</h2>
   *
   * <p>Every node in the parse tree is a {@link org.apache.calcite.sql.SqlNode}.
   *     Sub-types are:
   *
   * <ul>
   *
   * <li>{@link org.apache.calcite.sql.SqlLiteral} represents a boolean,
   *     numeric, string, or date constant, or the value <code>NULL</code>.
   *     </li>
   *
   * <li>{@link org.apache.calcite.sql.SqlIdentifier} represents an
   *     identifier, such as <code> EMPNO</code> or <code>emp.deptno</code>.
   *     </li>
   *
   * <li>{@link org.apache.calcite.sql.SqlCall} is a call to an operator or
   *     function.  By means of special operators, we can use this construct
   *     to represent virtually every non-leaf node in the tree. For example,
   *     a <code>select</code> statement is a call to the 'select'
   *     operator.</li>
   *
   * <li>{@link org.apache.calcite.sql.SqlNodeList} is a list of nodes.</li>
   *
   * </ul>
   *
   * <p>A {@link org.apache.calcite.sql.SqlOperator} describes the behavior of a
   *     node in the tree, such as how to un-parse a
   *     {@link org.apache.calcite.sql.SqlCall} into a SQL string.  It is
   *     important to note that operators are metadata, not data: there is only
   *     one <code>SqlOperator</code> instance representing the '=' operator, even
   *     though there may be many calls to it.
   *
   * <p><code>SqlOperator</code> has several derived classes which make it easy to
   *     define new operators: {@link org.apache.calcite.sql.SqlFunction},
   *     {@link org.apache.calcite.sql.SqlBinaryOperator},
   *     {@link org.apache.calcite.sql.SqlPrefixOperator},
   *     {@link org.apache.calcite.sql.SqlPostfixOperator}.
   * And there are singleton classes for special syntactic constructs
   *     {@link org.apache.calcite.sql.SqlSelectOperator}
   *     and {@link org.apache.calcite.sql.SqlJoin.SqlJoinOperator}. (These
   *     special operators even have their own sub-types of
   *     {@link org.apache.calcite.sql.SqlCall}:
   *     {@link org.apache.calcite.sql.SqlSelect} and
   *     {@link org.apache.calcite.sql.SqlJoin}.)
   *
   * <p>A {@link org.apache.calcite.sql.SqlOperatorTable} is a collection of
   *     operators. By supplying your own operator table, you can customize the
   *     dialect of SQL without modifying the parser.
   *
   * <h2>Validation</h2>
   *
   * <p>{@link org.apache.calcite.sql.validate.SqlValidator} checks that
   *     a tree of {@link org.apache.calcite.sql.SqlNode}s is
   *     semantically valid. You supply a
   *     {@link org.apache.calcite.sql.SqlOperatorTable} to describe the available
   *     functions and operators, and a
   *     {@link org.apache.calcite.sql.validate.SqlValidatorCatalogReader} for
   *     access to the database's catalog.
   *
   * <h2>Generating SQL</h2>
   *
   * <p>A {@link org.apache.calcite.sql.SqlWriter} converts a tree of
   * {@link org.apache.calcite.sql.SqlNode}s into a SQL string. A
   * {@link org.apache.calcite.sql.SqlDialect} defines how this happens.
   */
  @DefaultQualifier(value = NonNull.class, locations = TypeUseLocation.FIELD)
  @DefaultQualifier(value = NonNull.class, locations = TypeUseLocation.PARAMETER)
  @DefaultQualifier(value = NonNull.class, locations = TypeUseLocation.RETURN)
  package org.apache.calcite.sql;
  
  import org.checkerframework.checker.nullness.qual.NonNull;
  import org.checkerframework.framework.qual.DefaultQualifier;
  import org.checkerframework.framework.qual.TypeUseLocation;
  