expand one, two or multiple arrays into rows (multiple columns)
expand one array
Convert one array to row. E.g. Convert the following
| speeds |
|---|
| [1,2,3,4] |
to
| speed |
|---|
| 1 |
| 2 |
| 3 |
| 4 |
with the following SQL
WITH t AS
(SELECT array[1,
2,
3,
4] AS speeds)
SELECT speed
FROM t
CROSS JOIN unnest(speeds) AS t(speed)
expand multiple arrays
Convert arrays to rows. E.g. Convert the following
| speeds | costs |
|---|---|
| [1,2,3,4] | [5,6,7,8] |
to
| speed | cost |
|---|---|
| 1 | 5 |
| 2 | 6 |
| 3 | 7 |
| 4 | 8 |
with the following SQL
WITH t AS
(SELECT array[1,
2,
3,
4] AS speeds, array[5,
6,
7,
8] AS costs)
SELECT speed,
cost
FROM t
CROSS JOIN unnest(speeds, costs) AS t(speed, cost)
Histogram with equal width
Given a raw table
| val |
|---|
| 1 |
| 6 |
| 2 |
| 11 |
and then makes a histogram with width as 5. val_interval: 0 means there are 2 values (which are 1, 2) locating at the interval [0,4]. val_interval: 5, means there is 1 value (which is 6) locating at the interval [5,9].
| val_interval | total_count |
|---|---|
| 0 | 2 |
| 5 | 1 |
| 10 | 1 |
with the SQL
SELECT val / 5 * 5 AS val_interval,
count(1) AS total_count
FROM t
GROUP BY val / 5 * 5
ORDER BY val / 5 * 5
Numeric Histogram in Presto
Given the input table
| speeds |
|---|
| [1,2,3,4,5] |
firstly, let’s use the SQL to convert it to table t
| speed |
|---|
| 1 |
| 2 |
| 5 |
| 8 |
| 9 |
with the SQL
SELECT numeric_histogram(2,speed)
FROM t
CROSS JOIN unnest(speeds) AS t(speed)
and then, let’s make a numeric histogram, which separates the table into 2 buckets, and we get the result
| speed_interval | total_count |
|---|---|
| 2.67 | 3.00 |
| 8.50 | 2.00 |
with the SQL
WITH t AS
(SELECT array[1,
2,
5,
8,
9] AS speeds)
SELECT speed_interval,
total_count
FROM
(SELECT numeric_histogram(2,speed)
FROM t
CROSS JOIN unnest(speeds) AS t(speed)) AS x(hist)
CROSS JOIN UNNEST(hist) AS t (speed_interval, total_count)
Zip, filter in Presto
SQL
SELECT filter(cast(zip(array[1,2,3,4], array[5,6,7,8]) AS array<Row(speeds bigint, costs bigint)>), x -> x.speeds >= 3) as filtered
and we get
| filtered |
|---|
| [[3,7],[4,8]] |