quote:

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All rows in tbl_2 have a match in tbl_1

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SELECT f1 FROM tbl_2 EXCEPT SELECT f1 FROM tbl_1

quote:

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Originally posted by sslyle

I wish included these details.
tbl_1 has 1183 rows
tbl_2 has 1031 rows

All rows in tbl_2 have a match in tbl_1.
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select * from tbl_2 where f1 in (select f1 from tbl_1) -> returns 1031 rows
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f1 is a identifer (think about customer_id here). Each value in f1 in both tables are unique.

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Hi 
As you have already mentioned that table tbl_2 has 1031 
this records are matched with tbl_1 records it means  
tbl_1 has 1183 but 1031 records are matched or common 
thats why your query returning total 1031 records from 
your table .

Now talk about your UPDATE statement :


update a
set a.f2 = b.f2
from tbl_1 a
join tbl_2 b
on b.f1 = a.f1

Now this query updates tbl_1 on joining b.f1 = a.f1 
and it is updating 1029 records .It means tbl_1 has 
total 1029 records which are matched with tbl_2 which 
has 1032 records.

Now Talk about your Select statement :
select *
from tbl_1 a
join tbl_2 b
on b.f1 = a.f2

Inner Join basically retrieve all common values 
and here it is retrieving 1032 records which are
 matched with 1029 records of tbl_1