Postgres - 将两列聚合成一个项目

Postgres - aggregate two columns into one item

我想在分组时将两列合并为一列"array"。

假设 table 像这样:

friends_map:
=================================
user_id    friend_id    confirmed
=================================
1          2            true
1          3            false
2          1            true
2          3            true
1          4            false

我想从这个 table 中 select 并按 user_id 分组并得到 friend_id 并确认为以逗号分隔的串联值。

目前我有这个:

SELECT user_id, array_agg(friend_id) as friends, array_agg(confirmed) as confirmed
FROM friend_map
WHERE user_id = 1
GROUP BY user_id

这让我:

=================================
user_id    friends      confirmed
=================================
1         [2,3,4]       [t, f, f]

如何获得:

=================================
user_id    friends     
=================================
1         [ [2,t], [3,f], [4,f] ]

您可以在将值输入 array_agg() 函数之前将它们连接在一起:

SELECT user_id, array_agg('[' || friend_id || ',' || confirmed || ']') as friends
FROM friends_map
WHERE user_id = 1
GROUP BY user_id

演示:SQL Fiddle

Postgres 9.5 你可以获得文本数组的数组:

SELECT user_id, array_agg(array[friend_id::text, confirmed::text])
FROM friend_map
WHERE user_id = 1
GROUP BY user_id;

 user_id |           array_agg            
---------+--------------------------------
       1 | {{2,true},{3,false},{4,false}}
(1 row)

或 int 数组数组:

SELECT user_id, array_agg(array[friend_id, confirmed::int])
FROM friend_map
WHERE user_id = 1
GROUP BY user_id;

 user_id |      array_agg      
---------+---------------------
       1 | {{2,1},{3,0},{4,0}}
(1 row) 
SELECT user_id, array_agg((friend_id, confirmed)) as friends
FROM friend_map
WHERE user_id = 1
GROUP BY user_id

user_id |           array_agg            
--------+--------------------------------
      1 | {"(2,true)","(3,false)","(4,false)"}

你可以避免多维数组的丑陋并使用一些支持混合数据类型的json:

SELECT user_id, json_agg(json_build_array(friend_id, confirmed)) AS friends 
    FROM friends_map 
    WHERE user_id = 1
    GROUP BY user_id

或者使用一些 key : value 对,因为 json 允许这样做,所以你的输出会更 语义 如果你喜欢的话:

SELECT user_id, json_agg(json_build_object(
        'friend_id', friend_id, 
        'confirmed', confirmed
    )) AS friends 
    FROM friends_map 
    WHERE user_id = 1
    GROUP BY user_id;

您可以使用:array[]

就这样做:

SELECT user_id, array[friend_id, confirmed]
FROM friend_map
WHERE user_id = 1
GROUP BY 1;