我正在尝试在postgresql上编写以下查询:
select name, author_id, count(1),
(select count(1)
from names as n2
where n2.id = n1.id
and t2.author_id = t1.author_id
)
from names as n1
group by name, author_id
这肯定适用于Microsoft SQL Server,但它在postegresql上完全不起作用。我稍微阅读了它的文档,似乎我可以将其重写为:
select name, author_id, count(1), total
from names as n1, (select count(1) as total
from names as n2
where n2.id = n1.id
and n2.author_id = t1.author_id
) as total
group by name, author_id
但是在postegresql上返回以下错误:“FROM中的子查询不能引用相同查询级别的其他关系”。所以我被卡住了。有谁知道我怎么能做到这一点?
由于
答案 0 :(得分:98)
我不确定我是否理解你的意图,但也许以下内容会接近你想要的内容:
select n1.name, n1.author_id, count_1, total_count
from (select id, name, author_id, count(1) as count_1
from names
group by id, name, author_id) n1
inner join (select id, author_id, count(1) as total_count
from names
group by id, author_id) n2
on (n2.id = n1.id and n2.author_id = n1.author_id)
不幸的是,这增加了按id以及name和author_id对第一个子查询进行分组的要求,我认为不需要。我不知道如何解决这个问题,因为你需要有id可以加入第二个子查询。也许其他人会提出更好的解决方案。
分享并享受。
答案 1 :(得分:12)
我只是在这里回答我需要的最终sql的格式化版本,基于我上面评论中发布的Bob Jarvis答案:
select n1.name, n1.author_id, cast(count_1 as numeric)/total_count
from (select id, name, author_id, count(1) as count_1
from names
group by id, name, author_id) n1
inner join (select author_id, count(1) as total_count
from names
group by author_id) n2
on (n2.author_id = n1.author_id)
答案 2 :(得分:6)
补充 @Bob Jarvis 和 @dmikam 回答,Postgres在您不使用LATERAL时执行一个好的计划,在模拟之下,两种情况下查询数据结果相同,但成本差异很大
表格结构
CREATE TABLE ITEMS (
N INTEGER NOT NULL,
S TEXT NOT NULL
);
INSERT INTO ITEMS
SELECT
(random()*1000000)::integer AS n,
md5(random()::text) AS s
FROM
generate_series(1,1000000);
CREATE INDEX N_INDEX ON ITEMS(N);
在没有JOIN
的子查询中使用GROUP BY
执行LATERAL
EXPLAIN
SELECT
I.*
FROM ITEMS I
INNER JOIN (
SELECT
COUNT(1), n
FROM ITEMS
GROUP BY N
) I2 ON I2.N = I.N
WHERE I.N IN (243477, 997947);
结果
Merge Join (cost=0.87..637500.40 rows=23 width=37)
Merge Cond: (i.n = items.n)
-> Index Scan using n_index on items i (cost=0.43..101.28 rows=23 width=37)
Index Cond: (n = ANY ('{243477,997947}'::integer[]))
-> GroupAggregate (cost=0.43..626631.11 rows=861418 width=12)
Group Key: items.n
-> Index Only Scan using n_index on items (cost=0.43..593016.93 rows=10000000 width=4)
使用LATERAL
EXPLAIN
SELECT
I.*
FROM ITEMS I
INNER JOIN LATERAL (
SELECT
COUNT(1), n
FROM ITEMS
WHERE N = I.N
GROUP BY N
) I2 ON 1=1 --I2.N = I.N
WHERE I.N IN (243477, 997947);
结果
Nested Loop (cost=9.49..1319.97 rows=276 width=37)
-> Bitmap Heap Scan on items i (cost=9.06..100.20 rows=23 width=37)
Recheck Cond: (n = ANY ('{243477,997947}'::integer[]))
-> Bitmap Index Scan on n_index (cost=0.00..9.05 rows=23 width=0)
Index Cond: (n = ANY ('{243477,997947}'::integer[]))
-> GroupAggregate (cost=0.43..52.79 rows=12 width=12)
Group Key: items.n
-> Index Only Scan using n_index on items (cost=0.43..52.64 rows=12 width=4)
Index Cond: (n = i.n)
我的Postgres版本为PostgreSQL 10.3 (Debian 10.3-1.pgdg90+1)
答案 3 :(得分:5)
我知道这已经过时了,但是自Postgresql 9.3以来,您可以选择使用关键字" LATERAL"在JOINS中使用RELATED子查询,因此问题中的查询看起来像:
SELECT
name, author_id, count(*), t.total
FROM
names as n1
INNER JOIN LATERAL (
SELECT
count(*) as total
FROM
names as n2
WHERE
n2.id = n1.id
AND n2.author_id = n1.author_id
) as t ON 1=1
GROUP BY
n1.name, n1.author_id