Resolved: related columns

Question

I did not understand a simple point, is it indispensable to write the related columns such as " where e.emp_no= t.emp_no" if "exists" only tries to check the existence of certain elements? I know that without it, the query does not work correctly. However, I do not understand the necessity of typing it as we have selected * in a few lines before.

Answer 1

Hi Arsalan!
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. The point is to exercise EXISTS. Please, watch the lecture one more time. Thank you!

Hope this helps.
Best,
Tsvetelin

Resolved: related columns

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