ydb/yql/essentials/docs/en/syntax/flatten.md

FLATTEN

FLATTEN BY {#flatten-by}

Converts 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 resulting from FLATTEN BY on 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 it is necessary to output statistics by cells from a container column (for example, via GROUP BY).

• When the cells in a container column store IDs from another table that you want to join with 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.
• In FLATTEN BY you can use both columns and arbitrary named expressions (unlike columns, AS is required in this case). To avoid grammatical ambiguities of the expression after FLATTEN BY, make sure to use parentheses with the following: ... FLATTEN BY (ListSkip(col, 1) AS col) ...
• 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 info %}

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-optional type.

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>>.

{% endnote %}

Specifying the container type {#flatten-by-specific-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, treating 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.

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);

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

Analogues of FLATTEN BY in other DBMS {#flatten-other-dmb}

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

FLATTEN COLUMNS {#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;