impala/docs/topics/impala_identifiers.xml

<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE concept PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
<concept id="identifiers">

  <title>Overview of Impala Identifiers</title>
  <titlealts audience="PDF"><navtitle>Identifiers</navtitle></titlealts>
  <prolog>
    <metadata>
      <data name="Category" value="Impala"/>
      <data name="Category" value="SQL"/>
      <data name="Category" value="Data Analysts"/>
      <data name="Category" value="Developers"/>
      <data name="Category" value="Querying"/>
      <data name="Category" value="Databases"/>
      <data name="Category" value="Tables"/>
    </metadata>
  </prolog>

  <conbody>

    <p>
      Identifiers are the names of databases, tables, or columns that you specify in a SQL statement. The rules for
      identifiers govern what names you can give to things you create, the notation for referring to names
      containing unusual characters, and other aspects such as case sensitivity.
    </p>

    <ul>
      <li>
        <p>
        The minimum length of an identifier is 1 character.
        </p>
      </li>

      <li>
        <p>
        The maximum length of an identifier is currently 128 characters except for column names which
        can contain 767 characters, enforced by the metastore database.
        </p>
      </li>

      <li>
        <p>
        An identifier must start with an alphanumeric or underscore character except for column names which
        can start with any unicode characters. Quoting the identifier with backticks has no effect on the allowed
        characters in the name.
        </p>
      </li>

      <li>
        <p>
        An identifier can contain only ASCII characters except for column names which can contain unicode characters.
        </p>
      </li>

      <li>
        <p>
        To use an identifier name that matches one of the Impala reserved keywords (listed in
        <xref href="impala_reserved_words.xml#reserved_words"/>), surround the identifier with <codeph>``</codeph>
        characters (backticks). Quote the reserved word even if it is part of a fully qualified name.
        The following example shows how a reserved word can be used as a column name if it is quoted
        with backticks in the <codeph>CREATE TABLE</codeph> statement, and how the column name
        must also be quoted with backticks in a query:
        </p>
<codeblock>[localhost:21000] > create table reserved (`data` string);

[localhost:21000] > select data from reserved;
ERROR: AnalysisException: Syntax error in line 1:
select data from reserved
       ^
Encountered: DATA
Expected: ALL, CASE, CAST, DISTINCT, EXISTS, FALSE, IF, INTERVAL, NOT, NULL, STRAIGHT_JOIN, TRUE, IDENTIFIER
CAUSED BY: Exception: Syntax error

[localhost:21000] > select reserved.data from reserved;
ERROR: AnalysisException: Syntax error in line 1:
select reserved.data from reserved
                ^
Encountered: DATA
Expected: IDENTIFIER
CAUSED BY: Exception: Syntax error

[localhost:21000] > select reserved.`data` from reserved;

[localhost:21000] >
</codeblock>

        <note type="important">
          Because the list of reserved words grows over time as new SQL syntax is added,
          consider adopting coding conventions (especially for any automated scripts
          or in packaged applications) to always quote all identifiers with backticks.
          Quoting all identifiers protects your SQL from compatibility issues if
          new reserved words are added in later releases.
        </note>

      </li>

      <li>
        <p>
        Impala identifiers are always case-insensitive. That is, tables named <codeph>t1</codeph> and
        <codeph>T1</codeph> always refer to the same table, regardless of quote characters. Internally, Impala
        always folds all specified table and column names to lowercase. This is why the column headers in query
        output are always displayed in lowercase.
        </p>
      </li>
    </ul>

    <p>
      See <xref href="impala_aliases.xml#aliases"/> for how to define shorter or easier-to-remember aliases if the
      original names are long or cryptic identifiers.
      <ph conref="../shared/impala_common.xml#common/aliases_vs_identifiers"/>
    </p>

    <p conref="../shared/impala_common.xml#common/views_vs_identifiers"/>
  </conbody>
</concept>