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Editorial Team
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Asked: 2026-05-12T09:54:48+00:00

I need SQL code that will identify possible duplicates in a table. Lets say

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I need SQL code that will identify possible duplicates in a table. Lets say my table has 4 columns:

  • ID (primary key)

  • Date1

  • Date2

  • GroupID

(Date1, Date2, GroupID) form a unique key.

This table gets populated with blocks of data at a time, and it often happens that a new block is loaded in that contains a number of records that are already in there. This fine as long as the unique key catches them. Unfortunately, sometimes Date1 is empty (or at least ‘1900/01/01’) either with the first or subsequent uploads.

So what I need is something to identify where the (Date2, GroupID) combination appear more than once and where for one of the records Date1 = ‘1900/01/01’

Thanks

Karl

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1 Answer

  1. Editorial Team

    bkm kind of has it, but the inner select can perform poorly on some databases.
    This is more straightforward:

    select t1.* from 
t as t1 left join t as t2 
on (t1.date2=t2.date2 and t1.groupid=t2.groupid)
where t1.id != t2.id and (t1.date1='1900/01/01' or t2.date2='1900/01/01')
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