我有一个大型数据集,我需要以两种不同的方式进行分组。我希望我能够运行一次查询,这样我就不必再运行两个查询了。
我想这可能是使用ROLLUP
或GROUPING SETS
,但我必须承认我不完全明白我如何使用它们。
这是我正在尝试做的一个基本的例子。我试图用一个查询回答的两个问题是:
CREATE TABLE IF NOT EXISTS tmp_users (
id INTEGER NOT NULL,
name TEXT NOT NULL,
created TIMESTAMP NOT NULL,
companyid INTEGER NOT NULL
);
INSERT INTO tmp_users (id, name, created, companyid)
VALUES
(1, 'Lindsay', '2019-01-01', 1),
(2, 'Michael', '2019-01-02', 1),
(3, 'Stan', '2019-01-04', 3),
(4, 'Gob', '2019-01-04', 1),
(5, 'Buster', '2019-01-01', 1),
(6, 'Lucille', '2019-01-03', 2),
(7, 'Sally', '2019-01-05', 3);
-- Get users created per day, per company
SELECT
DATE_TRUNC('DAY', created) AS created,
companyid,
COUNT(*) AS numberofusers
FROM tmp_users
GROUP BY
DATE_TRUNC('DAY', created),
companyid
ORDER BY DATE_TRUNC('DAY', created) DESC;
-- Users per company, with filter
SELECT
companyid,
COUNT(*) AS numberofusers
FROM tmp_users
GROUP BY
companyid
HAVING COUNT(*) > 1
ORDER BY COUNT(*) DESC;
grouping sets
可用于在单个Select中返回多个级别的聚合:
-- Get users created per day, per company
SELECT *
FROM
(
SELECT
DATE_TRUNC('DAY', created) AS created,
companyid,
Count(*) AS numberofusers,
Row_Number() -- instead of TOP n
Over (PARTITION BY CASE WHEN DATE_TRUNC('DAY', created) IS NULL THEN 0 ELSE 1 END
ORDER BY Count(*) DESC) AS rn
FROM tmp_users
GROUP BY GROUPING SETS
(
(DATE_TRUNC('DAY', created), companyid) -- daily data
,companyid -- company data
)
) AS dt
WHERE created IS NOT NULL -- all daily data
OR rn <= 5 -- plus the TOP 5 companies
ORDER BY created ASC NULLS FIRST;