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WHERE is always conceptually against the results of the join(s)..

Not sure where you read " the where clause was against the right table" but
the source is either wrong or was mis-read.

Charles


On Fri, Mar 12, 2021 at 7:12 AM Jon Paris <jon.paris@xxxxxxxxxxxxxx> wrote:

Thanks Peter - good explanation - and one that was sorely missing form any
and all of the explanations of the left outer join that I could find.

I did think of it as a possibility but the docs all said all rows from the
left table and the where clause was against the right table. I guess the
join is done first and then the where causes the resulting set to be
filtered.


Jon

On Mar 11, 2021, at 8:01 PM, Peter Dow <petercdow@xxxxxxxxx> wrote:

Just for fun, imagine that every record in table1 has a record in
table2. But because you only want event = '2103SM', not all records are
selected. What's your record count?

You are selecting all records from table1 except those that have a
matching record in table2 with event not = '2103SM'. If you changed your
WHERE to AND, it would do what you're expecting:

select count(*)
from table1 a left outer join table2 b
on a.email = b.email
and b.event = '2103SM'
order by attend;



On 3/11/2021 4:26 PM, Jon Paris wrote:
Can someone please explain how I can ever get a lower row count from a
left outer join than the number of rows that exist in the left table?

select count(*)
from table1 a left outer join table2 b
on a.email = b.email
where b.event = '2103SM'
order by attend;

That returns query returns a count of 848 rows. There are 858 rows in
table1! (left table). And just for fun an inner join also returns 848 rows.

Any ideas anyone? I thought it was an absolute that a left outer
always returned all rows in the left table.


Jon Paris


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