Oracle Datatype Conversions

Last week I gave a colleague a demonstration on why, if you have numbers stored in a character string you should convert them explicitly to numbers:

select ‘Yes’
from dual
where ‘1000’ between ‘000’ and ‘999’;


Concatenate Two Fields And Get A Smaller Answer !?!?

Had a “strange” problem last week that really stumped me for longer than it should have.

My colleague had run a query to list some data and was getting approximately 200,000 rows back. We were investigating why a unique constraint was being violated so I took his query and did a count distinct to try and find the duplicates. I got 30,000 distinct values.

This really confused me, I was expecting only a handfull of duplicates, if any. Then I noticed that the first query already WAS a distinct list !

select distinct col1_id, col2_id from tablea

select count(distinct col1_id + ‘-‘ + col2_id) from tablea

I concatenated the fields together so I am counting a single field, I included the separator so that I could differentiate 1-11 from 11-1.

See if you can spot what the problem is before you read on.

Continue reading Concatenate Two Fields And Get A Smaller Answer !?!?

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