From the February 2016 Issue of Prestwood eMag

DBA ANSI SQL Scripting: Finding Duplicate Values Using SQL By Mike Prestwood

Posted 22 years ago on 11/19/2002 and updated 12/16/2008 Take Away: To find duplicate values in the same field, you need to use an alias with the same table twice.  Tags: SQL , duplicates KB100061

Finding Duplicate Values

To find duplicate values in the same field, you need to use an alias with the same table twice. Perhaps seeing this is easier than explaining it. Notice in the following example that the customer table is added twice to the "from" clause in the SQL statement. Then in the where clause both c1 and c2 aliases are used to refer to the same table. First we use both aliases to mach up the City column. Then we use both aliases to exclude the primary key.

Finding Duplicates in a single column:

select c1.*
from "customer.db" c1, "customer.db" c2
where c1."City" = c2."City"
and c1."Customer No" <> c2."Customer No"

Finding Duplicates in Multiple Fields:

select c1.*
from "customer.db" c1, "customer.db" c2
where c1."City" = c2."City"

and c1."State" = c2."State"
and c1."Customer No" <> c2."Customer No"

For multiple column primary key fields, you would add a line similar to the last line for each column.

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KB Post Contributed By Mike Prestwood:

Mike Prestwood is a drummer, an author, and creator of the PrestwoodBoards online community. He is the President & CEO of Prestwood IT Solutions. Prestwood IT provides Coding, Website, and Computer Tech services. Mike has authored 6 computer books and over 1,200 articles. As a drummer, he maintains play-drums.com and has authored 3 drum books. If you have a project you wish to discuss with Mike, you can send him a private message through his PrestwoodBoards home page or call him 9AM to 4PM PST at 916-726-5675 x205. Visit Profile