Usage of AND vs WHERE clause in SQL

Question

I am working on PHP page that connects with Oracle. I came across this SQL and I am not sure it is doing what is supposed to, so I thought I would

Answer 1

Am I right?

No. This query:

select tableA.id, tableA.name, tableB.details 
from tableA
left join tableB
on tableB.id = tableA.id
and tableB.logId = '<logged_in_user>'

Is very different from:

select tableA.id, tableA.name, tableB.details 
from tableA
left join tableB
on tableB.id = tableA.id
where tableB.logId = '<logged_in_user>'

It's the criteria for joining.

In the first case, you take from A, then join with B when there's a matching id and a logId, else leave details null.

In the second, you take from A, then join with B when there's a matching id, else leave details null, and then you keep only rows from A where there's a B match that has a logId -- eliminating rows from A in the process, and de facto turning it into an inner join.

Answer 2

If you only want results with values in both tables what you really want is an inner join. Once you have that, it doesn't really matter where you filter results out.

Answer 3

Now when I log in as a user who does not have entries in tableB, I am still getting records when this query runs.

Simply change the query to use an inner join like this:

select tableA.id, tableA.name, tableB.details 
from tableA
inner join tableB ...

and here is the definition of left join from Oracle:

The LEFT JOIN (also called LEFT OUTER JOIN) keyword returns all rows from the left table (table_name1), even if there are no matches in the right table (table_name2).

whereas the definition of the inner join is:

The INNER JOIN keyword return rows when there is at least one match in both tables.