[英]SQL query to select one row from each group with preference
例如,假設我有一個包含兩列X
和Y
的表。 現在,我要從該表中select
一些具有以下規則的行:
Y
列相同值的一組行。 X=a
,其中a是預定義的,則從該組中選擇該行。 X=a
,那么可以選擇任何行。 在單個SQL查詢中可以執行哪些操作?
嘗試這個:
SELECT DISTINCT
t1.Y,
COALESCE(a2.x, a1.x) 'x'
FROM Tablename t1
LEFT JOIN
(
SELECT Y, MIN(x) x
FROM Tablename
GROUP BY Y
) a1 ON t1.Y = a1.Y
LEFT JOIN
(
SELECT Y, MIN(x) x
FROM Tablename
WHERE x = 'a'
GROUP BY Y
) a2 ON t1.Y = a2.Y;
DROP SCHEMA tmp CASCADE;
CREATE SCHEMA tmp ;
SET search_path=tmp;
CREATE TABLE yx
( Y INTEGER NOT NULL
, x CHAR NOT NULL
, PRIMARY KEY (y,x)
);
INSERT INTO yx (y,x) VALUES
(10,'b') ,(10,'d') ,(10,'e' ) -- y==10 does not have an 'a'
,(20,'b') ,(20,'a') ,(20,'d') -- y==20 does have an 'a'
;
--
-- using a CTE
--
WITH this AS (
SELECT y,x
, row_number() OVER (PARTITION by y ORDER BY (x='a') DESC, x ASC) AS rnk
FROM yx
)
SELECT this.y,this.x
FROM this
WHERE this.rnk = 1
;
--
-- with a subselect instead of a CTE
--
SELECT this.y,this.x
FROM (
SELECT y,x
, row_number() OVER (PARTITION by y ORDER BY (x='a') DESC, x ASC) AS rnk
FROM yx
) this
WHERE this.rnk = 1
;
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