[ 
https://issues.apache.org/jira/browse/FLINK-4565?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16116473#comment-16116473
 ] 

ASF GitHub Bot commented on FLINK-4565:
---------------------------------------

Github user twalthr commented on a diff in the pull request:

    https://github.com/apache/flink/pull/4404#discussion_r131634081
  
    --- Diff: 
flink-libraries/flink-table/src/main/scala/org/apache/flink/table/expressions/subquery.scala
 ---
    @@ -0,0 +1,95 @@
    +/*
    + * 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.flink.table.expressions
    +
    +import com.google.common.collect.ImmutableList
    +import org.apache.calcite.rex.{RexNode, RexSubQuery}
    +import org.apache.calcite.sql.fun.SqlStdOperatorTable
    +import org.apache.calcite.tools.RelBuilder
    +import org.apache.flink.api.common.typeinfo.BasicTypeInfo._
    +import org.apache.flink.api.common.typeinfo.TypeInformation
    +import org.apache.flink.table.api.StreamTableEnvironment
    +import org.apache.flink.table.typeutils.TypeCheckUtils._
    +import org.apache.flink.table.validate.{ValidationFailure, 
ValidationResult, ValidationSuccess}
    +
    +case class In(expression: Expression, elements: Seq[Expression]) extends 
Expression  {
    +
    +  override def toString = s"$expression.in(${elements.mkString(", ")})"
    +
    +  override private[flink] def children: Seq[Expression] = expression +: 
elements.distinct
    +
    +  override private[flink] def toRexNode(implicit relBuilder: RelBuilder): 
RexNode = {
    +    // check if this is a sub-query expression or an element list
    +    elements.head match {
    +
    +      case TableReference(name, table) =>
    +        RexSubQuery.in(table.getRelNode, 
ImmutableList.of(expression.toRexNode))
    +
    +      case _ =>
    +        relBuilder.call(SqlStdOperatorTable.IN, children.map(_.toRexNode): 
_*)
    +    }
    +  }
    +
    +  override private[flink] def validateInput(): ValidationResult = {
    +    // check if this is a sub-query expression or an element list
    +    elements.head match {
    +
    +      case TableReference(name, table) =>
    +        if (elements.length != 1) {
    +          return ValidationFailure("IN operator supports only one table 
reference.")
    +        }
    +        if (table.tableEnv.isInstanceOf[StreamTableEnvironment]) {
    +          return ValidationFailure(
    +            "Sub-query IN operator on stream tables is currently not 
supported.")
    +        }
    +        val tableOutput = table.logicalPlan.output
    +        if (tableOutput.length > 1) {
    +          return ValidationFailure(
    +            s"The sub-query table '$name' must not have more than one 
column.")
    +        }
    +        (expression.resultType, tableOutput.head.resultType) match {
    +          case (lType, rType) if isNumeric(lType) && isNumeric(rType) => 
ValidationSuccess
    +          case (lType, rType) if lType == rType => ValidationSuccess
    --- End diff --
    
    Good point, yes we could do it. Feel free to submit a patch for it.


> Support for SQL IN operator
> ---------------------------
>
>                 Key: FLINK-4565
>                 URL: https://issues.apache.org/jira/browse/FLINK-4565
>             Project: Flink
>          Issue Type: Improvement
>          Components: Table API & SQL
>            Reporter: Timo Walther
>            Assignee: Timo Walther
>             Fix For: 1.4.0
>
>
> It seems that Flink SQL supports the uncorrelated sub-query IN operator. But 
> it should also be available in the Table API and tested.



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