Re: [GENERAL] Negating the list of selected rows of a join

Manuel Lemos <mlemos@acm.org> wrote:

 > I want to list the rows of a table with a text field whose values do not
 > exist in a similar field of another table.  Basically what I want to get
 > is negated results of a join. [...]
 > It worked except for the case when table_b is empty.  In this case the
 > nothing was returned.  Is this the expected behaviour or is it a bug in
 > PostgreSQL?

If you list two (or more) tables in the 'from' clause of a select
(that is, if you do a 'join'), a result table is built, in which each
row of the first table is combined with each row from (all) the other
table(s). To clarify, do simply

   SELECT table_a.name,table_b.name FROM table_a,table_b;

on your table. When one of the tables has no rows, all the rows from
the other(s) are combined with *nothing*; this gives nothing!
('combined' may be the wrong word; it's like a multiplication, and
people speak of a 'Cartesian product' of the tables)

The 'where' clause can restrict the rows of the result table to
something useful, e.g., you can restrict to 'table_a.name =
table_b.name'. A feature that probably will help you is the
construction of a so-called 'sub-select' in the where clause:

   SELECT name FROM table_a
   WHERE name NOT IN (SELECT name FROM table_b);

Hope it helps!
Ulf

--
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Ulf Mehlig    <umehlig@zmt.uni-bremen.de>
              Center for Tropical Marine Ecology/ZMT, Bremen, Germany
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