Return clickhouse array as column - sql

Question

Is it possible with Clickhouse to have result containing a pair of array transformed as columns?
Form this result:
┌─f1──┬f2───────┬f3─────────────┐
│ 'a' │ [1,2,3] │ ['x','y','z'] │
│ 'b' │ [4,5,6] │ ['x','y','z'] │
└─────┴─────────┴───────────────┘
to :
┌─f1──┬x──┬y──┬z──┐
│ 'a' │ 1 │ 2 │ 3 │
│ 'b' │ 4 │ 5 │ 6 │
└─────┴───┴───┴───┘
The idea is to not have to repeat the header values for each line.
In my case, the "header" array f3 unique by queries and join to the f1,f2.

Answer 1

You can do it with help of indexOf function.
SELECT *
FROM test_sof
┌─f1─┬─f2──────┬─f3────────────┐
│ a │ [1,2,3] │ ['x','y','z'] │
└────┴─────────┴───────────────┘
┌─f1─┬─f2────────┬─f3────────────────┐
│ c │ [7,8,9,0] │ ['x','y','z','n'] │
└────┴───────────┴───────────────────┘
┌─f1─┬─f2─────────┬─f3────────────────┐
│ d │ [7,8,9,11] │ ['x','y','z','n'] │
└────┴────────────┴───────────────────┘
┌─f1─┬─f2──────┬─f3────────────┐
│ b │ [4,5,6] │ ['x','y','z'] │
└────┴─────────┴───────────────┘
4 rows in set. Elapsed: 0.001 sec.
Then:
SELECT
f1,
f2[indexOf(f3, 'x')] AS x,
f2[indexOf(f3, 'y')] AS y,
f2[indexOf(f3, 'z')] AS z,
f2[indexOf(f3, 'n')] AS n
FROM test_sof
ORDER BY
f1 ASC,
x ASC
┌─f1─┬─x─┬─y─┬─z─┬──n─┐
│ a │ 1 │ 2 │ 3 │ 0 │
│ b │ 4 │ 5 │ 6 │ 0 │
│ c │ 7 │ 8 │ 9 │ 0 │
│ d │ 7 │ 8 │ 9 │ 11 │
└────┴───┴───┴───┴────┘
4 rows in set. Elapsed: 0.002 sec.
Keep in mind situation when index from header array will not be present in data array or vise-versa.
UPD: the way how to get data without knowing "headers".
You will get three columns, third one with headers.
SELECT
f1,
f2[num] AS f2_el,
f3[num] AS f3_el
FROM test_sof
ARRAY JOIN arrayEnumerate(f2) AS num
ORDER BY f1 ASC
┌─f1─┬─f2_el─┬─f3_el─┐
│ a │ 1 │ x │
│ a │ 2 │ y │
│ a │ 3 │ z │
│ b │ 4 │ x │
│ b │ 5 │ y │
│ b │ 6 │ z │
│ c │ 7 │ x │
│ c │ 8 │ y │
│ c │ 9 │ z │
│ c │ 0 │ n │
│ d │ 7 │ x │
│ d │ 8 │ y │
│ d │ 9 │ z │
│ d │ 11 │ n │
└────┴───────┴───────┘
14 rows in set. Elapsed: 0.006 sec.

Answer 2

This a fun puzzle. As pointed out already the indexOf() function seems to be the best way to pivot array columns inside ClickHouse but requires explicit selection of array positions. If you are using Python and your result set is not absurdly large, you can solve the problem in a more general way by flipping the array values into rows in SQL, then pivoting columns f2 and f3 in Python. Here's how it works.
First, use clickHouse-sqlalchemy and pandas to expand the matching arrays into rows as follows. (This example uses Jupyter Notebook running on Anaconda.)
# Load SQL Alchemy and connect to ClickHouse
from sqlalchemy import create_engine
%load_ext sql
%sql clickhouse://default:#localhost/default
# Use JOIN ARRAY to flip corresponding positions in f2, f3 to rows.
result = %sql select * from f array join f2, f3
df = result.DataFrame()
print(df)
The data frame appears as follows:
f1 f2 f3
0 a 1 x
1 a 2 y
2 a 3 z
3 b 4 x
4 b 5 y
5 b 6 z
Now we can pivot f2 and f3 into a new data frame.
dfp = df.pivot(columns='f3', values='f2', index='f1')
print(dfp)
The new dataframe dfp appears as follows:
f3 x y z
f1
a 1 2 3
b 4 5 6
This solution requires you to work outside the database but has the advantage that it works generally for any set of arrays as long as the names and values match. For instance if we add another row with different values and properties the same code gets the right answer. Here's a new row.
insert into f values ('c', [7,8,9,10], ['x', 'y', 'aa', 'bb'])
The pivoted data frame will appear as follows. NaN corresponds to missing values.
f3 aa bb x y z
f1
a NaN NaN 1.0 2.0 3.0
b NaN NaN 4.0 5.0 6.0
c 9.0 10.0 7.0 8.0 NaN
For more information on this solution see https://pandas.pydata.org/pandas-docs/stable/getting_started/dsintro.html and https://github.com/xzkostyan/clickhouse-sqlalchemy.