Z enim poljem zlahka najdete podvojitve:
SELECT name, COUNT(email)
FROM users
GROUP BY email
HAVING COUNT(email) > 1
Če imamo torej tabelo
ID NAME EMAIL
1 John [email protected]
2 Sam [email protected]
3 Tom [email protected]
4 Bob [email protected]
5 Tom [email protected]
Ta poizvedba nam bo dala John, Sam, Tom, Tom, ker imajo vsi isti email
.
Vendar pa želim dobiti podvojene osebe z istim naslovom
in imenom
.
To pomeni, da želim dobiti "Tom", "Tom".
Razlog, zakaj to potrebujem: Naredil sem napako in dovolil vstaviti podvojene vrednosti name
in email
. Zdaj moram podvojene vrednosti odstraniti/spremeniti, zato jih moram najprej najti.
SELECT
name, email, COUNT(*)
FROM
users
GROUP BY
name, email
HAVING
COUNT(*) > 1
Preprosto združite oba stolpca.
Opomba: po starejšem standardu ANSI naj bi bili v GROUP BY vsi stolpci, ki niso združeni, vendar se je to spremenilo z idejo "funkcionalne odvisnosti":
V teoriji relacijskih podatkovnih baz je funkcionalna odvisnost omejitev med dvema nizoma atributov v relaciji iz podatkovne baze. Z drugimi besedami, funkcionalna odvisnost je omejitev, ki opisuje razmerje med atributi v relaciji.
Podpora ni dosledna:
sql_mode=only_full_group_by
:poskusite to:
declare @YourTable table (id int, name varchar(10), email varchar(50))
INSERT @YourTable VALUES (1,'John','John-email')
INSERT @YourTable VALUES (2,'John','John-email')
INSERT @YourTable VALUES (3,'fred','John-email')
INSERT @YourTable VALUES (4,'fred','fred-email')
INSERT @YourTable VALUES (5,'sam','sam-email')
INSERT @YourTable VALUES (6,'sam','sam-email')
SELECT
name,email, COUNT(*) AS CountOf
FROM @YourTable
GROUP BY name,email
HAVING COUNT(*)>1
OUTPUT:
name email CountOf
---------- ----------- -----------
John John-email 2
sam sam-email 2
(2 row(s) affected)
če želite ID dvojnikov, uporabite to:
SELECT
y.id,y.name,y.email
FROM @YourTable y
INNER JOIN (SELECT
name,email, COUNT(*) AS CountOf
FROM @YourTable
GROUP BY name,email
HAVING COUNT(*)>1
) dt ON y.name=dt.name AND y.email=dt.email
OUTPUT:
id name email
----------- ---------- ------------
1 John John-email
2 John John-email
5 sam sam-email
6 sam sam-email
(4 row(s) affected)
za brisanje podvojitev poskusite:
DELETE d
FROM @YourTable d
INNER JOIN (SELECT
y.id,y.name,y.email,ROW_NUMBER() OVER(PARTITION BY y.name,y.email ORDER BY y.name,y.email,y.id) AS RowRank
FROM @YourTable y
INNER JOIN (SELECT
name,email, COUNT(*) AS CountOf
FROM @YourTable
GROUP BY name,email
HAVING COUNT(*)>1
) dt ON y.name=dt.name AND y.email=dt.email
) dt2 ON d.id=dt2.id
WHERE dt2.RowRank!=1
SELECT * FROM @YourTable
IZHODIŠČE:
id name email
----------- ---------- --------------
1 John John-email
3 fred John-email
4 fred fred-email
5 sam sam-email
(4 row(s) affected)