(no subject)

[den]

bloody SQL assignment is killing me.

"List all data from the Person table who are niether staff nor patients."

fucked if I know.

Why does

WHERE person.person_id = staff.person_id
AND person.person_id = admission.patient_id

work and

WHERE person.person_id <> staff.person_id
AND person.person_id <> admission.patient_id

not?

Comments

[charles.livejournal.com]

To answer the question.

Imagine the world's most fucking insane DBA came up with the following tables:

Now, try the first select: "Select first_name, last_name from table_1, table_2 where table_1.id = table_2.id"

Starting at the first row. We have table_1.id = 1, and first_name = 'Charles'. Looking across to the second table, we find all the rows where table_1.id = table_2.id. There's only one of those rows, and that joins me across to my last name, 'Miller'. So the results table will be:

Now for the second query. "Select first_name, last_name from table_1, table_2 where table_1.id <> table_2.id"

First, we look at the first row. The first_name is 'Charles', and the id is 1. Now we look at the second table, and find our join is telling us "find all the rows where table_1.id is NOT the same as table_2.id." The first row doesn't match this condition, but the other two do. So the result of this query would be:

[dewhitton.livejournal.com]

Kilkennys for you!