Re: SQL Gurus ...

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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