Resolved: EXISTS mechanism

Hi Marwa!
Thanks for reaching out.

If you run this query:

SELECT 
    *
FROM
    employees e
WHERE
    EXISTS( SELECT 
            *
        FROM
            titles
        WHERE
            title = 'Assistant Engineer');
the result will be all rows. Why? Because the condition in the subquery is always true. But we want to limit only these results which have title="Assistant Engineer". So what we need? Some line of code which will ensure we can get only these records. So, please use:

SELECT 
    *
FROM
    employees e
WHERE
    EXISTS( SELECT 
            *
        FROM
            titles t
        WHERE
            t.emp_no = e.emp_no
                AND title = 'Assistant Engineer');
This query will give a result set with 15 128 rows. Also the following query will give also 15 128 rows:

SELECT 
    *
FROM
    titles
WHERE
    title = 'Assistant Engineer';
But because we use in our query EXISTS we need to limit the result set with this additional line of code.

Hope this helps.
Best,
Tsvetelin