我有以下代码:
DROP SCHEMA IF EXISTS s CASCADE;
CREATE SCHEMA s;
CREATE TABLE "s"."t1"
(
"c1" BigSerial PRIMARY KEY,
"c2" BigInt NOT NULL,
"c3" BigInt
)
WITH (OIDS=FALSE);
INSERT INTO s.t1 (c2) VALUES (10);
INSERT INTO s.t1 (c2, c3) VALUES (20, 10);
INSERT INTO s.t1 (c2, c3) VALUES (30, 10);
/* 1. */ SELECT t3.c1 FROM s.t1 as t3 JOIN s.t1 as t2 ON t3.c3 = t2.c2;
/* 2. */ SELECT t1.c1, ARRAY_TO_STRING(ARRAY_AGG((t2.c1)), ',') FROM s.t1 LEFT JOIN s.t1 as t2
ON t2.c3 = t1.c2 GROUP BY t1.c1;
/* 3. */ SELECT c1, c2,
ARRAY_TO_STRING(ARRAY_AGG((SELECT t3.c1 FROM s.t1 as t3 JOIN s.t1 as t2 ON t3.c3 = t2.c2)), ',')
FROM s.t1 t1
GROUP BY c1;
DROP SCHEMA s CASCADE;
1 个查询的输出:
c1
----
2
3
(2 rows)
2 查询:
c1 | array_to_string
----+-----------------
1 | 2,3
2 |
3 |
(3 rows)
3 查询给我一个错误:
psql:/tmp/aggregate.sql:24: ERROR: more than one row returned by a subquery used as an expression
3 查询使用 1 查询作为内部查询。有没有办法让查询 3 与内部查询一起作为 1 而不是恢复为 2。
3 输出应与 2 相同。
我知道错误消息说查询 1 当用作 3 的子查询时不能返回多于一行。
请原谅我对数据库的了解有限。
回答:
SELECT c1, c2,
ARRAY_TO_STRING((SELECT ARRAY_AGG(t2.c1) FROM s.t1 as t2 WHERE t2.c3 = t1.c2), ',')
FROM s.t1 t1
GROUP BY c1;
最佳答案
array_agg
函数是一个聚合函数,需要用在列上,而不是集合上。
试试这个:
/* 3. */ SELECT c1, c2,
ARRAY_TO_STRING((SELECT ARRAY_AGG(t3.c1) FROM s.t1 as t3 JOIN s.t1 as t2 ON t3.c3 = t2.c2), ',')
FROM s.t1 t1
GROUP BY c1;
或者使用string_agg
函数:
/* 3. */ SELECT c1, c2,
(SELECT STRING_AGG(t3.c1::text, ',') FROM s.t1 as t3 JOIN s.t1 as t2 ON t3.c3 = t2.c2)
FROM s.t1 t1
GROUP BY c1;
关于mysql - 在内部查询中使用 array_agg 和数组字符串,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/37287867/