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FLATTEN

FLATTEN BY

Converts the rows in the source table using vertical unpacking of containers of variable length (lists or dictionaries).

For example:

  • Source table:
    |[a, b, c]|1|
    | --- | --- |
    |[d]|2|
    |[]|3|

  • The table after applying FLATTEN BY to the left column:
    |a|1|
    | --- | --- |
    |b|1|
    |c|1|
    |d|2|

Example

$sample = AsList(
    AsStruct(AsList('a','b','c') AS value, CAST(1 AS Uint32) AS id),
    AsStruct(AsList('d') AS value, CAST(2 AS Uint32) AS id),
    AsStruct(AsList() AS value, CAST(3 AS Uint32) AS id)
);


SELECT value, id FROM as_table($sample) FLATTEN BY (value);

This conversion can be convenient in the following cases:

  • When you need to output the statistics on cells from a container column (for example, by using GROUP BY).
  • When the cells in a container column store IDs from another table that you want to join using JOIN.

Syntax

  • FLATTEN BY is specified after FROM, but before GROUP BY, if GROUP BY is present in the query.
  • The type of the result column depends on the type of the source column:
Container type Result type Comments
List<X> X List cell type
Dict<X,Y> Tuple<X,Y> Tuple of two elements containing key-value pairs
Optional<X> X The result is almost equivalent to the clause WHERE foo IS NOT NULL, but the foo column type is changed to X
  • By default, the result column replaces the source column. Use FLATTEN BY foo AS bar to keep the source container. As a result, the source container is still available as foo and the output container is available as bar.
  • To build a Cartesian product of multiple container columns, use the clause FLATTEN BY (a, b, c). Parentheses are mandatory to avoid grammar conflicts.
  • Inside FLATTEN BY, you can only use column names from the input table. To apply FLATTEN BY to the calculation result, use a subquery.
  • If the source column had nested containers, for example, List<DictX,Y>, FLATTEN BY unpacks only the outer level. To completely unpack the nested containers, use a subquery.

Note

FLATTEN BY interprets optional data types as lists of length 0 or 1. The table rows with NULL are skipped, and the column type changes to a similar non-nullable one.

FLATTEN BY makes only one conversion at a time, so use FLATTEN LIST BY or FLATTEN OPTIONAL BY on optional containers, for example, Optional<List<String>>.

Specifying the container type

To specify the type of container to convert to, you can use:

  • FLATTEN LIST BY

    For Optional<List<T>>, FLATTEN LIST BY will unpack the list, interpreting NULL as an empty list.

  • FLATTEN DICT BY

    For Optional<Dict<T>>, FLATTEN DICT BY will unpack the dictionary, interpreting NULL as an empty dictionary.

  • FLATTEN OPTIONAL BY

    To filter the NULL values without serialization, specify the operation by using FLATTEN OPTIONAL BY.

Analogs of FLATTEN BY in other DBMS

  • PostgreSQL: unnest
  • Hive: LATERAL VIEW
  • MongoDB: unwind
  • Google BigQuery: FLATTEN
  • ClickHouse: ARRAY JOIN / arrayJoin

Examples

SELECT
  t.item.0 AS key,
  t.item.1 AS value,
  t.dict_column AS original_dict,
  t.other_column AS other
FROM my_table AS t
FLATTEN DICT BY dict_column AS item;

SELECT * FROM (
    SELECT
        AsList(1, 2, 3) AS a,
        AsList("x", "y", "z") AS b
) FLATTEN LIST BY (a, b);

FLATTEN COLUMNS

Converts a table where all columns must be structures to a table with columns corresponding to each element of each structure from the source columns.

The names of the source column structures are not used and not returned in the result. Be sure that the structure element names aren't repeated in the source columns.

Example

SELECT x, y, z
FROM (
  SELECT
    AsStruct(
        1 AS x,
        "foo" AS y),
    AsStruct(
        false AS z)
) FLATTEN COLUMNS;
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