How to Quickly Flatten a SQL Table

问题

I'm using Presto. If I have a table like:

ID    CATEGORY     VALUE
1      a           ...
1      b
1      c
2      a
2      b
3      b 
3      d
3      e
3      f

How would you convert to the below without writing a case statement for each combination?

ID      A     B    C    D    E    F
1
2
3

回答1:

I've never used Presto and the documentation seems pretty thin, but based on this article it looks like you could do

SELECT
  id,
  kv['A'] AS A,
  kv['B'] AS B,
  kv['C'] AS C,
  kv['D'] AS D,
  kv['E'] AS E,
  kv['F'] AS F
FROM (
  SELECT id, map_agg(category, value) kv
  FROM vtable
  GROUP BY id
) t

Although I'd recommend doing this in the display layer if possible since you have to specify the columns. Most reporting tools and UI grids support some sort of dynamic pivoting that will create columns based on the source data.

回答2:

My 2 cents:

If you know "possible" values:

SELECT 
    m['web'] AS web,
    m['shopping'] AS shopping,
    m['news'] AS news,
    m['music'] AS music,
    m['images'] AS images,
    m['videos'] AS videos,
    m[''] AS empty 
FROM (
SELECT histogram(data_tab) AS m
FROM datahub
WHERE
    year = 2017
    AND month = 5
    AND day = 7
    AND name = 'search'
) searches

No PIVOT function (yet)!

来源：https://stackoverflow.com/questions/37737358/how-to-quickly-flatten-a-sql-table