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Unions for Dummies

Posted on 2011-03-10
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Last Modified: 2012-05-11
I created a query that involved a bunch of joins.  Had a real SQL pro look at the query results and apparently they included too much.  The pro redid the query using a UNION.

Can someone explain to me what the data would look like in general for the pro to use a join?

What are the steps in deciding what select statements on each side of the join would look like?  I guess I'm asking for "how to do a union" explanation.
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Question by:lcor
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slightwv (䄆 Netminder) earned 500 total points
ID: 35098594
The simple on a straight UNION.

Take the unique results from: Give me everything from queryA add it to the results of give me everything from queryB.

A "UNION ALL" doesn't take unique records.

The caveats:
The number of columns must match and the datatypes for each individual column across the multiple queries must match.

You can do:
select 'string1' col1 , to_number(1) col2 from dual
union
select 'string2' col1 , to_number(2) col2 from dual

You cannot do:

select 'string1' col1 , to_number(1) col2 from dual
union
select to_number(2) col1 , 'string2' col2 from dual

The data types don't match.
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