sdks/java/extensions/sql/zetasql/src/main/java/org/apache/beam/sdk/extensions/sql/zetasql/SqlNullIfOperatorRewriter.java

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 * distributed with this work for additional information
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 * 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.beam.sdk.extensions.sql.zetasql;

import java.util.List;
import org.apache.beam.vendor.calcite.v1_20_0.org.apache.calcite.rex.RexBuilder;
import org.apache.beam.vendor.calcite.v1_20_0.org.apache.calcite.rex.RexNode;
import org.apache.beam.vendor.calcite.v1_20_0.org.apache.calcite.sql.SqlOperator;
import org.apache.beam.vendor.calcite.v1_20_0.org.apache.calcite.sql.fun.SqlStdOperatorTable;
import org.apache.beam.vendor.guava.v26_0_jre.com.google.common.base.Preconditions;
import org.apache.beam.vendor.guava.v26_0_jre.com.google.common.collect.ImmutableList;

/**
 * Rewrites NULLIF calls as CASE ($case_no_value) calls.
 *
 * <p>Turns <code>NULLIF(expression, expression_to_match)</code> into: <code><pre>CASE
 *   WHEN expression == expression_to_match THEN NULL
 *   ELSE expression
 *   END</pre></code>
 */
public class SqlNullIfOperatorRewriter implements SqlOperatorRewriter {
  @Override
  public RexNode apply(RexBuilder rexBuilder, List<RexNode> operands) {
    Preconditions.checkArgument(
        operands.size() == 2, "NULLIF should have two arguments in function call.");

    SqlOperator op =
        SqlStdOperatorMappingTable.ZETASQL_FUNCTION_TO_CALCITE_SQL_OPERATOR.get("$case_no_value");
    List<RexNode> newOperands =
        ImmutableList.of(
            rexBuilder.makeCall(
                SqlStdOperatorTable.EQUALS, ImmutableList.of(operands.get(0), operands.get(1))),
            rexBuilder.makeNullLiteral(operands.get(1).getType()),
            operands.get(0));

    return rexBuilder.makeCall(op, newOperands);
  }
}