Hive/Presto/标准SQL实现行转列和列转行

     假设有以下两个表格，分别为vtable和htable，期望使用SQL实现相互转化，本文将展示如何分别使用标准SQL、Presto、和Hive实现。

vtable

uid	key	value
101	c1	11
101	c2	12
101	c3	13
102	c1	21
102	c2	22
102	c3	23

htable

uid	c1	c2	c3
101	11	12	13
102	21	22	23

一、标准SQL实现

1.列转行(htable => vtable)

SELECT uid,
   max(CASE WHEN key = 'c1' THEN value END) AS c1,
   max(CASE WHEN key = 'c2' THEN value END) AS c2,
   max(CASE WHEN key = 'c3' THEN value END) AS c3
FROM vtable
GROUP BY uid

 2. 行转列(vtable => htable)

SELECT uid, 'c1' AS key, c1 AS value FROM htable
UNION ALL
SELECT uid, 'c2' AS key, c2 AS value FROM htable
UNION ALL
SELECT uid, 'c3' AS key, c3 AS value FROM htable

二、Hive实现

1. 列转行(vtable => htable)

SELECT
  uid,
  kv['c1'] AS c1,
  kv['c2'] AS c2,
  kv['c3'] AS c3
FROM (
  ----一般情况下是用',' ':'分隔符生成{k1:v1,k2:v2}样式的map
  ----这里使用\002,\003作为分割符可以防止key value中的 , :影响map的正确分隔
  SELECT uid,
    str_to_map(concat_ws('\002', collect_list(concat(key, '\003', value))), '\002', '\003') kv
  FROM vtable
  GROUP BY uid
) t

2. 行转列(htable => vtable)

SELECT t1.uid, t2.key, t2.value
FROM htable t1
LATERAL VIEW explode (map(
  'c1', c1,
  'c2', c2,
  'c3', c3
)) t2 as key, value

三、Presto实现

 1. 列转行(vtable => htable)

SELECT
  uid,
  kv['c1'] AS c1,
  kv['c2'] AS c2,
  kv['c3'] AS c3
FROM (
  SELECT uid, map_agg(key, value) kv
  FROM vtable
  GROUP BY uid
) t

2. 行转列(htable => vtable)

SELECT t1.uid, t2.key, t2.value
FROM htable t1
CROSS JOIN unnest (
  array['c1', 'c2', 'c3'],
  array[c1, c2, c3]
) t2 (key, value)

关于PRESTO的UNNEST

UNNEST 用于将 ARRAY 或 MAP 扩展到关系表中。ARRAY中得元素会被放到一个单独的列中，而MAP中的元素会被放到两个列中(key, value)。 UNNEST可以选择性的添加WITH ORDINALITY语句，这样可以在关系表的后面增加一个序号列。

1. 单个数组列:

SELECT student, score
FROM (
  VALUES
    ('LiMing', ARRAY[20, 50]),
    ('WangGang', ARRAY[70,80,90])
) AS x (student, scores)
CROSS JOIN UNNEST(scores) AS t (score);

  student  | score
-----------+---
 LiMing    | 20 
 LiMing    | 50 
 WangGang  | 70 
 WangGang  | 80 
 WangGang  | 90
(5 rows)

2. 多个数组列:

SELECT numbers, animals, n, a
FROM (
  VALUES
    (ARRAY[2, 5], ARRAY['dog', 'cat', 'bird']),
    (ARRAY[7, 8, 9], ARRAY['cow', 'pig'])
) AS x (numbers, animals)
CROSS JOIN UNNEST(numbers, animals) AS t (n, a);

  numbers  |     animals      |  n   |  a
-----------+------------------+------+------
 [2, 5]    | [dog, cat, bird] |    2 | dog
 [2, 5]    | [dog, cat, bird] |    5 | cat
 [2, 5]    | [dog, cat, bird] | NULL | bird
 [7, 8, 9] | [cow, pig]       |    7 | cow
 [7, 8, 9] | [cow, pig]       |    8 | pig
 [7, 8, 9] | [cow, pig]       |    9 | NULL
(6 rows)

3. WITH ORDINALITY 语句:

SELECT numbers, n, a
FROM (
  VALUES
    (ARRAY[2, 5]),
    (ARRAY[7, 8, 9])
) AS x (numbers)
CROSS JOIN UNNEST(numbers) WITH ORDINALITY AS t (n, a);

  numbers  | n | a
-----------+---+---
 [2, 5]    | 2 | 1
 [2, 5]    | 5 | 2
 [7, 8, 9] | 7 | 1
 [7, 8, 9] | 8 | 2
 [7, 8, 9] | 9 | 3
(5 rows)

4. Map+ARRAY:

SELECT
    animals, a, n
FROM (
    VALUES
        (MAP(ARRAY['dog', 'cat', 'bird'], ARRAY[1, 2, 0])),
        (MAP(ARRAY['dog', 'cat'], ARRAY[4, 5]))
) AS x (animals)
CROSS JOIN UNNEST(animals) AS t (a, n);

           animals          |  a   | n
----------------------------+------+---
 {"cat":2,"bird":0,"dog":1} | dog  | 1
 {"cat":2,"bird":0,"dog":1} | cat  | 2
 {"cat":2,"bird":0,"dog":1} | bird | 0
 {"cat":5,"dog":4}          | dog  | 4
 {"cat":5,"dog":4}          | cat  | 5
(5 rows)