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

  <title>UPDATE Statement (<keyword keyref="impala28"/> or higher only)</title>
  <titlealts audience="PDF"><navtitle>UPDATE</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">UPDATE statement</indexterm>
      Updates an arbitrary number of rows in 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>
UPDATE [<varname>database_name</varname>.]<varname>table_name</varname> SET <varname>col</varname> = <varname>val</varname> [, <varname>col</varname> = <varname>val</varname> ... ]
  [ FROM <varname>joined_table_refs</varname> ]
  [ WHERE <varname>where_conditions</varname> ]
</codeblock>

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

    <p>
      None of the columns that make up the primary key can be updated by the
      <codeph>SET</codeph> clause.
    </p>

    <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>
      If the <codeph>WHERE</codeph> clause is omitted, all rows in the table are updated.
    </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>
      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 updated 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>
          An <codeph>UPDATE</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 matching rows than expected
          in the table because it is undefined whether the <codeph>UPDATE</codeph> applies to rows
          that are inserted or updated while the <codeph>UPDATE</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>
      The optional <codeph>FROM</codeph> clause lets you restrict the
      updates to only the rows in the specified table that are part
      of the result set for a join query. The join clauses can include
      non-Kudu tables, but the table from which the rows are deleted
      must be a Kudu table.
    </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 perform a simple update
      on a table, with or without a <codeph>WHERE</codeph> clause:
    </p>

<codeblock>
-- Set all rows to the same value for column c3.
-- In this case, c1 and c2 are primary key columns
-- and so cannot be updated.
UPDATE kudu_table SET c3 = 'not applicable';

-- Update only the rows that match the condition.
UPDATE kudu_table SET c3 = NULL WHERE c1 > 100 AND c3 IS NULL;

-- Does not update any rows, because the WHERE condition is always false.
UPDATE kudu_table SET c3 = 'impossible' WHERE 1 = 0;

-- Change the values of multiple columns in a single UPDATE statement.
UPDATE kudu_table SET c3 = upper(c3), c4 = FALSE, c5 = 0 WHERE c6 = TRUE;
</codeblock>

    <p>
      The following examples show how to perform an update using the
      <codeph>FROM</codeph> keyword with a join clause:
    </p>

<codeblock>
-- Uppercase a column value, only for rows that have
-- an ID that matches the value from another table.
UPDATE kudu_table SET c3 = upper(c3)
  FROM kudu_table JOIN non_kudu_table
  ON kudu_table.id = non_kudu_table.id;

-- Same effect as previous statement.
-- Assign table aliases in FROM clause, then refer to
-- short names elsewhere in the statement.
UPDATE t1 SET c3 = upper(c3)
  FROM kudu_table t1 JOIN non_kudu_table t2
  ON t1.id = t2.id;

-- Same effect as previous statements, but more efficient.
-- Use WHERE clause to skip updating values that are
-- already uppercase.
UPDATE t1 SET c3 = upper(c3)
  FROM kudu_table t1 JOIN non_kudu_table t2
  ON t1.id = t2.id
  WHERE c3 != upper(c3);
</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_delete.xml#delete"/>, <xref href="impala_upsert.xml#upsert"/>
    </p>

  </conbody>

</concept>