SQL Server cross apply not working?

Question

http://sqlfiddle.com/#!3/78273/1

create table emptb1
(
id int,
name varchar(20),
dept int
)

insert into emptb1 values (1,'vish',10);
insert into emptb1 values (2,'vish',10);
insert into emptb1 values (3,'vish',30);
insert into emptb1 values (4,'vish',20);

create table depttb1
(
id int,
name varchar(20)
)

insert into depttb1 values(10,'IT')
insert into depttb1 values(20,'AC')
insert into depttb1 values(30,'LIC')

select * from emptb1

select e.id, e.name, a.id
from emptb1 e
cross apply
(
select top 1 * from depttb1 d
where d.id = e.dept
order by d.id desc
) a

I was trying to learn cross apply as it's similar as inner join but works with function.

In above query I'm assuming it should take only dept=30 because order d.id desc will give only top 1st id which is 30 and then it should return employees with dept id = 30 but it's giving me all the rows and all the deptid.

What's wrong with query or I'm wrong interpreting the concept of cross apply.

Answer 1

You say "In above query I'm assuming it should take only dept=30 because order d.id desc will give only top 1st id which is 30 and then it should return employees with dept id = 30".

That's not how it works. Here's your query (reformatted a little for clarity):

select e.id, e.name, a.id
from   emptb1 e
cross apply
(
    select top 1 * 
    from depttb1 d
    where d.id = e.dept
    order by d.id desc
) a

The APPLY keyword means that the inner query is (logically) called once for each row of the outer query. For what happens inside the inner query, it's helpful to understand the logical order that the clauses of a SELECT are executed in. This order is:

1. FROM clause
2. WHERE clause
3. SELECT columns
4. ORDER BY clause
5. TOP operator

Note that in your inner query then, the TOP operator gets applied last, well after the WHERE clause. This means the where d.id = e.dept will first reduce the inner rows to those whose d.id matches the e.dept of the outer row (which is not necessarily 30), then sort them, and then return the first one. And it does this for every row in the outer query. So obviously, many of them are not going to be 30.

What you are trying to would be more akin to this (still retaining the CROSS APPLY):

select e.id, e.name, a.id
from   emptb1 e
cross apply
(
    select top 1 * 
    from
    (
        select top 1 * 
        from depttb1 d
        order by d.id desc
    ) b
    where b.id = e.dept
) a

Here, the logic has been reordered by use of another, nested, sub-query that insures that the ORDER BY, then TOP 1 get applied before the WHERE clause. (Note that this would not normally the recommended way to do this as nested sub-queries can hamper readability, I just used it here to retain the CROSS APPLY and to retain the rest of the original structure).

Answer 2

To exand on Damien's comment, the inner query:

select top 1 * from depttb1 d
where d.id = e.dept
order by d.id desc

is going to run for every row in the outer query:

select e.id, e.name, a.id
from emptb1 e

So you will always get a match from the inner query for each row. I think you were expecting the inner query to run only one time, but that's not what APPLY does.

So, taking the first row from your outer query, with an ID of 1 and a dept id of 10, your inner query will translate to:

select top 1 * from depttb1 d
where d.id = 10  //this is the dept id for the current row from your outer query
order by d.id desc