Mega Code Archive
Find duplicate rows in a table [no primary key]
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Question:
I have a table of city names and need to find duplicate entries. The table does not have a primary key, so the duplicate rows are entirely identical.
Answer:
You can use the GROUP clause in your SELECT statement. Below are 3 different attempts. Try the first one first. It may not work with your DBMS but if it works, it's most convenient. The second one is an ok choice as it returns all rows but the duplicate rows in the 'See Also' box.
The last version returns all city names with a count value and you manually have to look for those with a value > 1. If you need to do this more often, you should consider creating a VIEW or a temporary table with this query and then do a SELECT on this VIEW/ temp table.
In Microsoft SQL-Server, you can use the HAVING clause, as shown at the bottom.
And: If you do have a primary key defined, see the other tip mentioned at the top. This would work in that situation, but using the unique ID is better.
// Return all duplicate cities and how often they appear
// Works with ORACLE
select city_name, count(city_name) as cnt
from areas
group by city_name
where cnt>1
// not all SQL dbms will support the reference to the count column cnt in the where clause.
// The following will return ALL rows with counter, but sorted by number of appearances
// Your duplicates will be at the top.
// Works with MYSQL
select city_name, count(city_name) as cnt
from areas
group by city_name
order by cnt desc
// finally, no back reference to count column cnt at all-
// the following will work on all SQL dbms:
// Return all cities and how often they appear
select city_name, count(city_name) as cnt
from areas
group by city_name
// version for Micrsoft's MSSQL Server
// make use of the HAVING clause
select city_name
from areas
group by city_name
having count(*) > 1
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