sql - 在 group by 子句中连接数组

Question

我们在将数组分组为单个数组时遇到了问题。 我们想将两列的值连接到一个数组中，然后聚合这些多行数组。

给定以下输入:

| id | name | col_1 | col_2 |
| 1  |  a   |   1   |   2   |
| 2  |  a   |   3   |   4   |
| 4  |  b   |   7   |   8   |
| 3  |  b   |   5   |   6   |

我们想要以下输出:

| a | { 1, 2, 3, 4 } |
| b | { 5, 6, 7, 8 } |

元素的顺序很重要，应该与聚合行的 ID 相关联。

我们尝试了 array_agg() 函数:

SELECT array_agg(ARRAY[col_1, col_2]) FROM mytable GROUP BY name;

不幸的是，这个语句引发了一个错误:

ERROR: could not find array type for data type character varying[]

似乎不可能使用 array_agg() 将数组合并到一个 group by 子句中。

有什么想法吗？

Answer 1

联合所有

您可以先使用 UNION ALL 进行“反转”:

SELECT name, array_agg(c) AS c_arr
FROM  (
   SELECT name, id, 1 AS rnk, col1 AS c FROM tbl
   UNION ALL
   SELECT name, id, 2, col2 FROM tbl
   ORDER  BY name, id, rnk
   ) sub
GROUP  BY 1;

适用于生成您稍后请求的值的顺序。 The manual:

The aggregate functions array_agg, json_agg, string_agg, and xmlagg, as well as similar user-defined aggregate functions, produce meaningfully different result values depending on the order of the input values. This ordering is unspecified by default, but can be controlled by writing an ORDER BY clause within the aggregate call, as shown in Section 4.2.7. Alternatively, supplying the input values from a sorted subquery will usually work.

大胆强调我的。

LATERAL subquery使用 VALUES 表达式

LATERAL 需要 Postgres 9.3 或更高版本。

SELECT t.name, array_agg(c) AS c_arr
FROM  (SELECT * FROM tbl ORDER BY name, id) t
CROSS  JOIN LATERAL (VALUES (t.col1), (t.col2)) v(c)
GROUP  BY 1;

同样的结果。只需要在 table 上走一遍。

自定义聚合函数

或者您可以创建自定义聚合函数，就像这些相关答案中讨论的那样:

• Selecting data into a Postgres array
• Is there something like a zip() function in PostgreSQL that combines two arrays?

CREATE AGGREGATE array_agg_mult (anyarray)  (
    SFUNC     = array_cat
  , STYPE     = anyarray
  , INITCOND  = '{}'
);

然后你可以:

SELECT name, array_agg_mult(ARRAY[col1, col2] ORDER BY id) AS c_arr
FROM   tbl
GROUP  BY 1
ORDER  BY 1;

或者，通常更快，但不是标准 SQL:

SELECT name, array_agg_mult(ARRAY[col1, col2]) AS c_arr
FROM  (SELECT * FROM tbl ORDER BY name, id) t
GROUP  BY 1;

添加的 ORDER BY id(可以附加到此类聚合函数)保证您想要的结果:

a | {1,2,3,4}
b | {5,6,7,8}

或者您可能对这个替代方案感兴趣:

SELECT name, array_agg_mult(ARRAY[ARRAY[col1, col2]] ORDER BY id) AS c_arr
FROM   tbl
GROUP  BY 1
ORDER  BY 1;

生成二维数组:

a | {{1,2},{3,4}}
b | {{5,6},{7,8}}

最后一个可以用 Postgres 9.5 或更高版本中的内置 array_agg() 替换(并且应该替换，因为它更快!) - 添加聚合数组的能力:

SELECT name, array_agg(ARRAY[col1, col2] ORDER BY id) AS c_arr
FROM   tbl
GROUP  BY 1
ORDER  BY 1;

同样的结果。 The manual:

input arrays concatenated into array of one higher dimension (inputs must all have same dimensionality, and cannot be empty or null)

所以和我们自定义的聚合函数array_agg_mult()不完全一样；