FLATTEN
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
YDB tables don't support container types, so the FLATTEN BY function can only be applied to table-type variables created within a YQL query.
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 afterFROM
, but beforeGROUP BY
, ifGROUP 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 asfoo
and the output container is available asbar
. - 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 applyFLATTEN 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 afterFLATTEN 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
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>>
.
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, treatingNULL
as an empty list. -
FLATTEN DICT BY
For
Optional<Dict<T>>
,FLATTEN DICT BY
will unpack the dictionary, interpretingNULL
as an empty dictionary. -
FLATTEN OPTIONAL BY
To filter the
NULL
values without serialization, specify the operation by usingFLATTEN 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
- PostgreSQL:
unnest
- Hive:
LATERAL VIEW
- MongoDB:
unwind
- Google BigQuery:
FLATTEN
- ClickHouse:
ARRAY JOIN / arrayJoin
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;