EXPLAIN EXTENDED

From Stack Overflow:

Can anyone give me a good example of when CROSS APPLY makes a difference in those cases where INNER JOIN will work as well?

This is of course SQL Server.

A quick reminder on the terms.

INNER JOIN is the most used construct in SQL: it joins two tables together, selecting only those row combinations for which a JOIN condition is true.

This query:

SELECT  *
FROM    table1
JOIN    table2
ON      table2.b = table1.a

reads:

For each row from table1, select all rows from table2 where the value of field b is equal to that of field a

Note that this condition can be rewritten as this:

SELECT  *
FROM    table1, table2
WHERE   table2.b = table1.a

, in which case it reads as following:

Make a set of all possible combinations of rows from table1 and table2 and of this set select only the rows where the value of field b is equal to that of field a

These conditions are worded differently, but they yield the same result and database systems are aware of that. Usually both these queries are optimized to use the same execution plan.

The former syntax is called ANSI syntax, and it is generally considered more readable and is recommended to use.

However, it didn't make it into Oracle until recently, that's why there are many hardcore Oracle developers that are just used to the latter syntax.

Actually, it's a matter of taste.

To use JOINs (with whatever syntax), both sets you are joining must be self-sufficient, i. e. the sets should not depend on each other. You can query both sets without ever knowing the contents on another set.

But for some tasks the sets are not self-sufficient. For instance, let's consider the following query:

We have table1 and table2. table1 has a column called rowcount.

For each row from table1 we need to select first rowcount rows from table2, ordered by table2.id

We cannot come up with a join condition here. The join condition, should it exist, would involve the row number, which is not present in table2, and there is no way to calculate a row number only from the values of columns of any given row in table2.

That's where the CROSS APPLY can be used.

CROSS APPLY is a Microsoft's extension to SQL, which was originally intended to be used with table-valued functions (TVF's).

The query above would look like this:

SELECT  *
FROM    table1
CROSS APPLY
(
SELECT  TOP (table1.rowcount) *
FROM    table2
ORDER BY
id
) t2

For each from table1, select first table1.rowcount rows from table2 ordered by id

The sets here are not self-sufficient: the query uses values from table1 to define the second set, not to JOIN with it.

The exact contents of t2 are not known until the corresponding row from table1 is selected.

I previously said that there is no way to join these two sets, which is true as long as we consider the sets as is. However, we can change the second set a little so that we get an additional computed field we can later join on.

The first option to do that is just count all preceding rows in a subquery:

SELECT  *
FROM    table1 t1
JOIN    (
        SELECT  t2o.*,
                (
                SELECT  COUNT(*)
                FROM    table2 t2i
                WHERE   t2i.id <= t2o.id
                ) AS rn
        FROM    table2 t2o
        ) t2
ON      t2.rn <= t1.rowcount

The second option is to use a window function, also available in SQL Server since version 2005:

SELECT  *
FROM    table1 t1
JOIN    (
        SELECT  t2o.*, ROW_NUMBER() OVER (ORDER BY id) AS rn
        FROM    table2 t2o
        ) t2
ON      t2.rn <= t1.rowcount

This function returns the ordinal number a row would have, be the ORDER BY condition used in the function applied to the whole query.

This is essentially the same result as the subquery used in the previous query.

Now, let's create the sample tables and check all these solutions for efficiency:

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