How to get average values from dictionary looking values using SQL? - mysql

Question

my dataframe looks like this:
id value
a 0:3,1:0,2:0,3:4
a 0:0,1:0,2:2,3:0
a 0:0,1:5,2:4,3:0
I want to write a query to get average values of keys in column value?
So for example for 0:3,1:0,2:0,3:4 it must be (0+0+0+3+3+3+3)/7 = 1.71.
For 0:0,1:0,2:2,3:0 it must be (2+2)/2=2.
For 0:0,1:5,2:4,3:0 it must be (1+1+1+1+1+2+2+2+2)/9 = 1.44.
So desired result is:
id value
a 1.71
a 2.00
a 1.44
How to do that? Are there sql functions to get this result?

Answer 1

See this DBFIDDLE
code:
CREATE PROCEDURE `avg_dict`(s varchar(100))
BEGIN
SET #result = CONCAT('SELECT (', replace(replace(s, ":","*"),",","+"), ')/(',regexp_replace(s,",?[0-9]:","+"),')');
PREPARE stmt FROM #result;
EXECUTE stmt ;
DEALLOCATE PREPARE stmt;
END
results:
stmt
output
CALL avg_dict("0:3,1:0,2:0,3:4");
1.1743
CALL avg_dict("0:0,1:0,2:2,3:0");
2.0000
CALL avg_dict("0:0,1:5,2:4,3:0");
1.4444

Answer 2

With some combination of split's, transforms and repeat you can achieve your goal:
WITH dataset(id, value) AS (
values ('a', '0:3,1:0,2:0,3:4'),
('a', '0:0,1:0,2:2,3:0'),
('a', '0:0,1:5,2:4,3:0')
)
SELECT id,
reduce(arr, 0.0, (s, x)->s + x, s->s) / cardinality(arr)
FROM(
SELECT *,
flatten(
transform(
transform(
split(value, ','),
s->split(s, ':')
),
arr->repeat(
cast(arr [ 1 ] as INTEGER),
cast(arr [ 2 ] as INTEGER)
)
)
) as arr
FROM dataset
)
Output:
id
_col1
a
1.7142857142857142
a
2.0
a
1.4444444444444444
Note:
Outer select can be substituted with array_average but I used he select cause Athena's version of Presto does not support it.
UPD
Another version which can be more performant:
SELECT id,
reduce(
arr,
CAST(ROW(0.0, 0) AS ROW(sum DOUBLE, count INTEGER)),
(s, r)->CAST(
ROW(r.num * r.count + s.sum, s.count + r.count) AS ROW(sum DOUBLE, count INTEGER)
),
s->IF(s.count = 0, NULL, s.sum / s.count)
)
FROM(
SELECT *,
transform(
split(value, ','),
s->CAST(
ROW(
CAST(split(s, ':') [ 1 ] AS DOUBLE),
(CAST(split(s, ':') [ 2 ] AS INTEGER))
) AS ROW(num DOUBLE, count INTEGER)
)
) as arr
FROM dataset
)