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<!DOCTYPE concept PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
<concept id="delete" rev="kudu">

  <title>DELETE Statement (<keyword keyref="impala28"/> or higher only)</title>
  <titlealts audience="PDF"><navtitle>DELETE</navtitle></titlealts>
  <prolog>
    <metadata>
      <data name="Category" value="Impala"/>
      <data name="Category" value="SQL"/>
      <data name="Category" value="Kudu"/>
      <data name="Category" value="ETL"/>
      <data name="Category" value="Ingest"/>
      <data name="Category" value="DML"/>
      <data name="Category" value="Developers"/>
      <data name="Category" value="Data Analysts"/>
    </metadata>
  </prolog>

  <conbody>

    <p>
      <indexterm audience="hidden">DELETE statement</indexterm>
      Deletes an arbitrary number of rows from a Kudu table.
      This statement only works for Impala tables that use the Kudu storage engine.
    </p>

    <p conref="../shared/impala_common.xml#common/syntax_blurb"/>

<codeblock>
DELETE [FROM] [<varname>database_name</varname>.]<varname>table_name</varname> [ WHERE <varname>where_conditions</varname> ]

DELETE <varname>table_ref</varname> FROM [<varname>joined_table_refs</varname>] [ WHERE <varname>where_conditions</varname> ]
</codeblock>

    <p>
      The first form evaluates rows from one table against an optional
      <codeph>WHERE</codeph> clause, and deletes all the rows that
      match the <codeph>WHERE</codeph> conditions, or all rows if
      <codeph>WHERE</codeph> is omitted.
    </p>

    <p>
      The second form evaluates one or more join clauses, and deletes
      all matching rows from one of the tables. The join clauses can
      include non-Kudu tables, but the table from which the rows
      are deleted must be a Kudu table. The <codeph>FROM</codeph>
      keyword is required in this case, to separate the name of
      the table whose rows are being deleted from the table names
      of the join clauses.
    </p>

    <p conref="../shared/impala_common.xml#common/usage_notes_blurb"/>

    <p>
      The conditions in the <codeph>WHERE</codeph> clause are the same ones allowed
      for the <codeph>SELECT</codeph> statement. See <xref href="impala_select.xml#select"/>
      for details.
    </p>

    <p>
      The conditions in the <codeph>WHERE</codeph> clause can refer to
      any combination of primary key columns or other columns. Referring to
      primary key columns in the <codeph>WHERE</codeph> clause is more efficient
      than referring to non-primary key columns.
    </p>

    <p>
      If the <codeph>WHERE</codeph> clause is omitted, all rows are removed from the table.
    </p>

    <p>
      Because Kudu currently does not enforce strong consistency during concurrent DML operations,
      be aware that the results after this statement finishes might be different than you
      intuitively expect:
    </p>
    <ul>
      <li>
        <p>
          If some rows cannot be deleted because their
          some primary key columns are not found, due to their being deleted
          by a concurrent <codeph>DELETE</codeph> operation,
          the statement succeeds but returns a warning.
        </p>
      </li>
      <li>
        <p>
          A <codeph>DELETE</codeph> statement might also overlap with
          <codeph>INSERT</codeph>, <codeph>UPDATE</codeph>,
          or <codeph>UPSERT</codeph> statements running concurrently on the same table.
          After the statement finishes, there might be more or fewer rows than expected in the table
          because it is undefined whether the <codeph>DELETE</codeph> applies to rows that are
          inserted or updated while the <codeph>DELETE</codeph> is in progress.
        </p>
      </li>
    </ul>

    <p>
      The number of affected rows is reported in an <cmdname>impala-shell</cmdname> message
      and in the query profile.
    </p>

    <p conref="../shared/impala_common.xml#common/dml_blurb_kudu"/>

    <note conref="../shared/impala_common.xml#common/compute_stats_next"/>

    <p conref="../shared/impala_common.xml#common/example_blurb"/>

    <p>
      The following examples show how to delete rows from a specified
      table, either all rows or rows that match a <codeph>WHERE</codeph>
      clause:
    </p>

<codeblock>
-- Deletes all rows. The FROM keyword is optional.
DELETE FROM kudu_table;
DELETE kudu_table;

-- Deletes 0, 1, or more rows.
-- (If c1 is a single-column primary key, the statement could only
-- delete 0 or 1 rows.)
DELETE FROM kudu_table WHERE c1 = 100;

-- Deletes all rows that match all the WHERE conditions.
DELETE FROM kudu_table WHERE
  (c1 > c2 OR c3 IN ('hello','world')) AND c4 IS NOT NULL;
DELETE FROM t1 WHERE
  (c1 IN (1,2,3) AND c2 > c3) OR c4 IS NOT NULL;
DELETE FROM time_series WHERE
  year = 2016 AND month IN (11,12) AND day > 15;

-- WHERE condition with a subquery.
DELETE FROM t1 WHERE
  c5 IN (SELECT DISTINCT other_col FROM other_table);

-- Does not delete any rows, because the WHERE condition is always false.
DELETE FROM kudu_table WHERE 1 = 0;
</codeblock>

    <p>
      The following examples show how to delete rows that are part
      of the result set from a join:
    </p>

<codeblock>
-- Remove _all_ rows from t1 that have a matching X value in t2.
DELETE t1 FROM t1 JOIN t2 ON t1.x = t2.x;

-- Remove _some_ rows from t1 that have a matching X value in t2.
DELETE t1 FROM t1 JOIN t2 ON t1.x = t2.x
  WHERE t1.y = FALSE and t2.z > 100;

-- Delete from a Kudu table based on a join with a non-Kudu table.
DELETE t1 FROM kudu_table t1 JOIN non_kudu_table t2 ON t1.x = t2.x;

-- The tables can be joined in any order as long as the Kudu table
-- is specified as the deletion target.
DELETE t2 FROM non_kudu_table t1 JOIN kudu_table t2 ON t1.x = t2.x;
</codeblock>

    <p conref="../shared/impala_common.xml#common/related_info"/>

    <p>
      <xref href="impala_kudu.xml#impala_kudu"/>, <xref href="impala_insert.xml#insert"/>,
      <xref href="impala_update.xml#update"/>, <xref href="impala_upsert.xml#upsert"/>
    </p>

  </conbody>

</concept>