impala/docs/topics/impala_array.xml

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

  <title>ARRAY Complex Type (<keyword keyref="impala23"/> or higher only)</title>

  <prolog>
    <metadata>
      <data name="Category" value="Impala"/>
      <data name="Category" value="Impala Data Types"/>
      <data name="Category" value="SQL"/>
      <data name="Category" value="Developers"/>
      <data name="Category" value="Data Analysts"/>
    </metadata>
  </prolog>

  <conbody>

    <p>
      A complex data type that can represent an arbitrary number of ordered elements.
      The elements can be scalars or another complex type (<codeph>ARRAY</codeph>,
      <codeph>STRUCT</codeph>, or <codeph>MAP</codeph>).
    </p>

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

<!-- To do: make sure there is sufficient syntax info under the SELECT statement to understand how to query all the complex types. -->

<codeblock><varname>column_name</varname> ARRAY &lt; <varname>type</varname> &gt;

type ::= <varname>primitive_type</varname> | <varname>complex_type</varname>
</codeblock>

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

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

      <p>
        The elements of the array have no names. You refer to the value of the array item using the
        <codeph>ITEM</codeph> pseudocolumn, or its position in the array with the <codeph>POS</codeph>
        pseudocolumn. See <xref href="impala_complex_types.xml#item"/> for information about
        these pseudocolumns.
      </p>

<!-- Array is a frequently used idiom; don't recommend MAP right up front, since that is more rarely used. STRUCT has all different considerations.
      <p>
        If it would be logical to have a fixed number of elements and give each one a name, consider using a
        <codeph>MAP</codeph> (when all elements are of the same type) or a <codeph>STRUCT</codeph> (if different
        elements have different types) instead of an <codeph>ARRAY</codeph>.
      </p>
-->

    <p>
      Each row can have a different number of elements (including none) in the array for that row.
    </p>

<!-- Since you don't use numeric indexes, this assertion and advice doesn't make sense.
      <p>
        If you attempt to refer to a non-existent array element, the result is <codeph>NULL</codeph>. Therefore,
        when using operations such as addition or string concatenation involving array elements, you might use
        conditional functions to substitute default values such as 0 or <codeph>""</codeph> in the place of missing
        array elements.
      </p>
-->

      <p>
        When an array contains items of scalar types, you can use aggregation functions on the array elements without using join notation. For
        example, you can find the <codeph>COUNT()</codeph>, <codeph>AVG()</codeph>, <codeph>SUM()</codeph>, and so on of numeric array
        elements, or the <codeph>MAX()</codeph> and <codeph>MIN()</codeph> of any scalar array elements by referring to
        <codeph><varname>table_name</varname>.<varname>array_column</varname></codeph> in the <codeph>FROM</codeph> clause of the query. When
        you need to cross-reference values from the array with scalar values from the same row, such as by including a <codeph>GROUP
        BY</codeph> clause to produce a separate aggregated result for each row, then the join clause is required.
      </p>

      <p>
        A common usage pattern with complex types is to have an array as the top-level type for the column:
        an array of structs, an array of maps, or an array of arrays.
        For example, you can model a denormalized table by creating a column that is an <codeph>ARRAY</codeph>
        of <codeph>STRUCT</codeph> elements; each item in the array represents a row from a table that would
        normally be used in a join query. This kind of data structure lets you essentially denormalize tables by
        associating multiple rows from one table with the matching row in another table.
      </p>

      <p>
        You typically do not create more than one top-level <codeph>ARRAY</codeph> column, because if there is
        some relationship between the elements of multiple arrays, it is convenient to model the data as
        an array of another complex type element (either <codeph>STRUCT</codeph> or <codeph>MAP</codeph>).
      </p>

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

      <p conref="../shared/impala_common.xml#common/added_in_230"/>

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

      <ul conref="../shared/impala_common.xml#common/complex_types_restrictions">
        <li/>
      </ul>

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

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

      <p>
        The following example shows how to construct a table with various kinds of <codeph>ARRAY</codeph> columns,
        both at the top level and nested within other complex types.
        Whenever the <codeph>ARRAY</codeph> consists of a scalar value, such as in the <codeph>PETS</codeph>
        column or the <codeph>CHILDREN</codeph> field, you can see that future expansion is limited.
        For example, you could not easily evolve the schema to record the kind of pet or the child's birthday alongside the name.
        Therefore, it is more common to use an <codeph>ARRAY</codeph> whose elements are of <codeph>STRUCT</codeph> type,
        to associate multiple fields with each array element.
      </p>

      <note>
        Practice the <codeph>CREATE TABLE</codeph> and query notation for complex type columns
        using empty tables, until you can visualize a complex data structure and construct corresponding SQL statements reliably.
      </note>

<!-- To do: verify and flesh out this example. -->

<codeblock><![CDATA[CREATE TABLE array_demo
(
  id BIGINT,
  name STRING,
-- An ARRAY of scalar type as a top-level column.
  pets ARRAY <STRING>,

-- An ARRAY with elements of complex type (STRUCT).
  places_lived ARRAY < STRUCT <
    place: STRING,
    start_year: INT
  >>,

-- An ARRAY as a field (CHILDREN) within a STRUCT.
-- (The STRUCT is inside another ARRAY, because it is rare
-- for a STRUCT to be a top-level column.)
  marriages ARRAY < STRUCT <
    spouse: STRING,
    children: ARRAY <STRING>
  >>,

-- An ARRAY as the value part of a MAP.
-- The first MAP field (the key) would be a value such as
-- 'Parent' or 'Grandparent', and the corresponding array would
-- represent 2 parents, 4 grandparents, and so on.
  ancestors MAP < STRING, ARRAY <STRING> >
)
STORED AS PARQUET;
]]>
</codeblock>

    <p>
      The following example shows how to examine the structure of a table containing one or more <codeph>ARRAY</codeph> columns by using the
      <codeph>DESCRIBE</codeph> statement. You can visualize each <codeph>ARRAY</codeph> as its own two-column table, with columns
      <codeph>ITEM</codeph> and <codeph>POS</codeph>.
    </p>

<!-- To do: extend the examples to include MARRIAGES and ANCESTORS columns, or get rid of those columns. -->

<codeblock><![CDATA[DESCRIBE array_demo;
+--------------+---------------------------+
| name         | type                      |
+--------------+---------------------------+
| id           | bigint                    |
| name         | string                    |
| pets         | array<string>             |
| marriages    | array<struct<             |
|              |   spouse:string,          |
|              |   children:array<string>  |
|              | >>                        |
| places_lived | array<struct<             |
|              |   place:string,           |
|              |   start_year:int          |
|              | >>                        |
| ancestors    | map<string,array<string>> |
+--------------+---------------------------+

DESCRIBE array_demo.pets;
+------+--------+
| name | type   |
+------+--------+
| item | string |
| pos  | bigint |
+------+--------+

DESCRIBE array_demo.marriages;
+------+--------------------------+
| name | type                     |
+------+--------------------------+
| item | struct<                  |
|      |   spouse:string,         |
|      |   children:array<string> |
|      | >                        |
| pos  | bigint                   |
+------+--------------------------+

DESCRIBE array_demo.places_lived;
+------+------------------+
| name | type             |
+------+------------------+
| item | struct<          |
|      |   place:string,  |
|      |   start_year:int |
|      | >                |
| pos  | bigint           |
+------+------------------+

DESCRIBE array_demo.ancestors;
+-------+---------------+
| name  | type          |
+-------+---------------+
| key   | string        |
| value | array<string> |
+-------+---------------+
]]>
</codeblock>

    <p>
      The following example shows queries involving <codeph>ARRAY</codeph> columns containing elements of scalar or complex types. You
      <q>unpack</q> each <codeph>ARRAY</codeph> column by referring to it in a join query, as if it were a separate table with
      <codeph>ITEM</codeph> and <codeph>POS</codeph> columns. If the array element is a scalar type, you refer to its value using the
      <codeph>ITEM</codeph> pseudocolumn. If the array element is a <codeph>STRUCT</codeph>, you refer to the <codeph>STRUCT</codeph> fields
      using dot notation and the field names. If the array element is another <codeph>ARRAY</codeph> or a <codeph>MAP</codeph>, you use
      another level of join to unpack the nested collection elements.
    </p>

<!-- To do: have some sample output to show for these queries. -->

<codeblock><![CDATA[-- Array of scalar values.
-- Each array element represents a single string, plus we know its position in the array.
SELECT id, name, pets.pos, pets.item FROM array_demo, array_demo.pets;

-- Array of structs.
-- Now each array element has named fields, possibly of different types.
-- You can consider an ARRAY of STRUCT to represent a table inside another table.
SELECT id, name, places_lived.pos, places_lived.item.place, places_lived.item.start_year
FROM array_demo, array_demo.places_lived;

-- The .ITEM name is optional for array elements that are structs.
-- The following query is equivalent to the previous one, with .ITEM
-- removed from the column references.
SELECT id, name, places_lived.pos, places_lived.place, places_lived.start_year
  FROM array_demo, array_demo.places_lived;

-- To filter specific items from the array, do comparisons against the .POS or .ITEM
-- pseudocolumns, or names of struct fields, in the WHERE clause.
SELECT id, name, pets.item FROM array_demo, array_demo.pets
  WHERE pets.pos in (0, 1, 3);

SELECT id, name, pets.item FROM array_demo, array_demo.pets
  WHERE pets.item LIKE 'Mr. %';

SELECT id, name, places_lived.pos, places_lived.place, places_lived.start_year
  FROM array_demo, array_demo.places_lived
WHERE places_lived.place like '%California%';
]]>
</codeblock>

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

    <p>
      <xref href="impala_complex_types.xml#complex_types"/>,
<!-- <xref href="impala_array.xml#array"/>, -->
      <xref href="impala_struct.xml#struct"/>, <xref href="impala_map.xml#map"/>
    </p>

  </conbody>

</concept>