SQL 查询以拆分并仅保留前 N 个值

Question

我有以下 table 数据:

| name  |items     |
--------------------
| Bob   |1, 2, 3   |
| Rick  |5, 3, 8, 4|
| Bill  |2, 4      |

我需要创建一个 table 并拆分 items 列，但限制为最多 N items 每个 name。例如。对于 N = 3，table 应如下所示：

|name |item|
-----------
|Bob  |1   |
|Bob  |2   |
|Bob  |3   |
|Rick |5   | 
|Rick |3   | 
|Rick |8   |
|Bill |2   |
|Bill |4   |

我有以下查询正确拆分 items，但没有考虑最大数量 N。我应该在查询中修改什么（标准 SQL , BigQuery)占N?

WITH data_split AS (
    SELECT name, SPLIT(items,',') AS item
    FROM (
        SELECT name, items
        -- A lot of additional logic here
        FROM data
    )
)

SELECT name, item
FROM data_split
CROSS JOIN UNNEST(data_split.item) AS item

Answer 1

您可以尝试一种更半标准的方式 - 几乎无处不在：

WITH
-- your input ...                                                                                                                                                                                   
indata(id,nam,items) AS (             -- need a sorting column "id" to keep the sort order
          SELECT 1, 'Bob' ,'1,2,3'    -- blanks after comma can irritate 
UNION ALL SELECT 2, 'Rick','5,3,8,4'  -- the splitting function below ...
UNION ALL SELECT 3, 'Bill','2,4'
)
-- real query starts here, replace comma below with "WITH" ...
,
-- exactly 3 integers
i(i) AS (
          SELECT 1   -- need to add FROM DUAL , in Oracle, for example ...
UNION ALL SELECT 2
UNION ALL SELECT 3
)
SELECT
  id
, nam
, SPLIT(items,',',i) AS item -- SPLIT_PART in other DBMS-s
FROM indata CROSS JOIN i 
WHERE SPLIT_PART(items,',',i) <> ''
ORDER BY 1, 3
;
-- out  id | nam  | item 
-- out ----+------+------
-- out   1 | Bob  | 1
-- out   1 | Bob  | 2
-- out   1 | Bob  | 3
-- out   2 | Rick | 3
-- out   2 | Rick | 5
-- out   2 | Rick | 8
-- out   3 | Bill | 2
-- out   3 | Bill | 4

Answer 2

考虑以下方法 (BigQuery)

select name, trim(item) item
from your_table, unnest(split(items)) item with offset 
where offset < 3

如果应用于您问题中的示例数据 - 输出为

SQL 查询以拆分并仅保留前 N 个值

SQL query to split and keep only the top N values

sql

split

google-bigquery