Removing duplicates in an SQL database on a condition

Question

I need to remove duplicates from a table that looks like this:

id           post_author    post_title
-----------------------------------------------------------------------------
21319        1              Youngstown State University
20535        1              Yo San University of Traditional Chinese Medicine
30268        29             Yo San University of Traditional Chinese Medicine
29747        29             Yeshiva University
21964        1              Yale University
29247        29             Yale University
29497        29             Xavier University of Louisiana
21916        1              Xavier University
29862        29             Xavier University
29860        29             Wright State University-Main Campus
20915        1              Wright State University-Lake Campus
21562        1              World Mission University
30267        29             World Mission University

Basically, if there are two entries with the same post_title, I need to remove the one with post_author = 1, but if the post_title is unique then the entry should be left as is.

How can this be done with an SQL query?

EDIT:

I've tried a query suggested by Mureinik. The query looks like this:

DELETE t FROM wp_posts AS t 
WHERE t.post_author = 1 AND
EXISTS (SELECT * FROM wp_posts s 
                WHERE t.post_title = s.post_title
                AND s.post_authot != 1)

But I got error:

[Err] 1093 - You can't specify target table 't' for update in FROM clause

What am I doing wrong?

is this select or delete??

Anonymous Duck
– Anonymous Duck

2015-05-15 10:08:33 +00:00
Commented May 15, 2015 at 10:08 — Anonymous Duck
– Anonymous Duck, Commented May 15, 2015 at 10:08
Is it on mysql or postgresql?

user_0
– user_0

2015-05-15 10:09:35 +00:00
Commented May 15, 2015 at 10:09 — user_0
– user_0, Commented May 15, 2015 at 10:09
It's delete, I need to remove duplicates. It's for MySQL

Victor Marchuk
– Victor Marchuk

2015-05-18 11:10:56 +00:00
Commented May 18, 2015 at 11:10 — Victor Marchuk
– Victor Marchuk, Commented May 18, 2015 at 11:10

Mureinik · Accepted Answer · 2015-05-15 10:08:30Z

2

You could use the exists operator:

DELETE FROM my_table t
WHERE  post_author = 1 AND
       EXISTS (SELECT *
               FROM   my_table s
               WHERE  t.post_title = s.post_title AND
                      s.post_author != 1)

answered May 15, 2015 at 10:08

Mureinik

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1 Comment

Victor Marchuk Over a year ago

I've tried this, but got an error. Please, take a look at the updated question. Any ideas on what I did wrong?

kiks73 · Accepted Answer · 2015-05-18 15:01:36Z

1

You could set a condition on post_author = 1 where there are more than one post_title with a temporary table:

CREATE TEMPORARY TABLE t_del AS SELECT post_title 
                     FROM t 
                     GROUP BY post_title 
                     HAVING count(post_title)>1;               

DELETE FROM t 
WHERE post_author = 1 
  AND post_title IN (select post_title FROM t_del) ;

SQL Fiddle here

edited May 18, 2015 at 15:01

answered May 15, 2015 at 10:12

kiks73

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1 Comment

kiks73 Over a year ago

You are right, you need a temporary table. This cannot be done in a single SQL. Check my updated answer

Giorgi Nakeuri · Accepted Answer · 2015-05-15 10:09:40Z

0

With in operator:

delete from TableName where post_author = 1
and post_title in(select post_title from TableName group by post_title having count(*) > 1)

answered May 15, 2015 at 10:09

Giorgi Nakeuri

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