How to keep only one row of a table, removing duplicate rows?

问题

I have a table that has a lot of duplicates in the Name column. I'd like to only keep one row for each.

The following lists the duplicates, but I don't know how to delete the duplicates and just keep one:

SELECT name FROM members GROUP BY name HAVING COUNT(*) > 1;

Thank you.

回答1:

See the following question: Deleting duplicate rows from a table.

The adapted accepted answer from there (which is my answer, so no "theft" here...):

You can do it in a simple way assuming you have a unique ID field: you can delete all records that are the same except for the ID, but don't have "the minimum ID" for their name.

Example query:

DELETE FROM members
WHERE ID NOT IN
(
    SELECT MIN(ID)
    FROM members
    GROUP BY name
)

In case you don't have a unique index, my recommendation is to simply add an auto-incremental unique index. Mainly because it's good design, but also because it will allow you to run the query above.

回答2:

It would probably be easier to select the unique ones into a new table, drop the old table, then rename the temp table to replace it.

#create a table with same schema as members
CREATE TABLE tmp (...);

#insert the unique records
INSERT INTO tmp SELECT * FROM members GROUP BY name;

#swap it in
RENAME TABLE members TO members_old, tmp TO members;

#drop the old one
DROP TABLE members_old;

回答3:

We have a huge database where deleting duplicates is part of the regular maintenance process. We use DISTINCT to select the unique records then write them into a TEMPORARY TABLE. After TRUNCATE we write back the TEMPORARY data into the TABLE.

That is one way of doing it and works as a STORED PROCEDURE.

回答4:

If we want to see first which rows you are about to delete. Then delete them.

with MYCTE as (
    SELECT DuplicateKey1
        ,DuplicateKey2 --optional
        ,count(*) X
    FROM MyTable
    group by DuplicateKey1, DuplicateKey2
    having count(*) > 1
) 
SELECT E.*
FROM MyTable E
JOIN MYCTE cte
ON E.DuplicateKey1=cte.DuplicateKey1
    AND E.DuplicateKey2=cte.DuplicateKey2
ORDER BY E.DuplicateKey1, E.DuplicateKey2, CreatedAt

Full example at http://developer.azurewebsites.net/2014/09/better-sql-group-by-find-duplicate-data/

回答5:

You can join table with yourself by matched field and delete unmatching rows

DELETE t1 FROM table_name t1 
LEFT JOIN tablename t2 ON t1.match_field = t2.match_field
WHERE t1.id <> t2.id;

回答6:

delete dup row keep one table has duplicate rows and may be some rows have no duplicate rows then it keep one rows if have duplicate or single in a table. table has two column id and name if we have to remove duplicate name from table and keep one. Its Work Fine at My end You have to Use this query.

DELETE FROM tablename
WHERE id NOT IN(

 SELECT id FROM
(
    SELECT MIN(id)AS id
    FROM tablename
    GROUP BY name HAVING 
    COUNT(*) > 1
)AS a )
AND id NOT IN(
(SELECT ids FROM
(
SELECT MIN(id)AS ids
    FROM tablename
    GROUP BY name HAVING 
    COUNT(*) =1
)AS a1
)
)

before delete table is below see the screenshot: enter image description here after delete table is below see the screenshot this query delete amit and akhil duplicate rows and keep one record (amit and akhil):

enter image description here

回答7:

if you want to remove duplicate record from table.

CREATE TABLE tmp SELECT lastname, firstname, sex
FROM user_tbl;
GROUP BY (lastname, firstname);

DROP TABLE user_tbl;

ALTER TABLE tmp RENAME TO user_tbl;

回答8:

WITH CTE AS
(
    SELECT ROW_NUMBER() OVER (PARTITION BY [emp_id] ORDER BY [emp_id]) AS Row, * FROM employee_salary
)


DELETE FROM CTE
WHERE ROW <> 1

来源：https://stackoverflow.com/questions/1286843/how-to-keep-only-one-row-of-a-table-removing-duplicate-rows

标签

sql

sqlite

duplicates