When should I use CROSS APPLY over INNER JOIN?

This has already been answered very well technically, but let me give a concrete example of how it's extremely useful:

Lets say you have two tables, Customer and Order. Customers have many Orders.

I want to create a view that gives me details about customers, and the most recent order they've made. With just JOINS, this would require some self-joins and aggregation which isn't pretty. But with Cross Apply, its super easy:

SELECT *
FROM Customer
CROSS APPLY (
SELECT TOP 1 *
FROM Order
WHERE Order.CustomerId = Customer.CustomerId
ORDER BY OrderDate DESC
) T

Consider you have two tables.

**MASTER TABLE**

x------x--------------------x
| Id | Name |
x------x--------------------x
| 1 | A |
| 2 | B |
| 3 | C |
x------x--------------------x

**DETAILS TABLE**

x------x--------------------x-------x
| Id | PERIOD | QTY |
x------x--------------------x-------x
| 1 | 2014-01-13 | 10 |
| 1 | 2014-01-11 | 15 |
| 1 | 2014-01-12 | 20 |
| 2 | 2014-01-06 | 30 |
| 2 | 2014-01-08 | 40 |
x------x--------------------x-------x

There are many situations where we need to replace `INNER JOIN` with `CROSS APPLY`.

**1. Join two tables based on `TOP n` results**

Consider if we need to select `Id` and `Name` from `Master` and last two dates for each `Id` from `Details table`.

SELECT M.ID,M.NAME,D.PERIOD,D.QTY
FROM MASTER M
INNER JOIN
(
SELECT TOP 2 ID, PERIOD,QTY
FROM DETAILS D
ORDER BY CAST(PERIOD AS DATE)DESC
)D
ON M.ID=D.ID

- **[SQL FIDDLE][1]**

The above query generates the following result.

x------x---------x--------------x-------x
| Id | Name | PERIOD | QTY |
x------x---------x--------------x-------x
| 1 | A | 2014-01-13 | 10 |
| 1 | A | 2014-01-12 | 20 |
x------x---------x--------------x-------x

See, it generated results for last two dates with last two date's `Id` and then joined these records only in the outer query on `Id`, which is wrong. This should be returning both `Ids` 1 and 2 but it returned only 1 because 1 has the last two dates. To accomplish this, we need to use `CROSS APPLY`.

SELECT M.ID,M.NAME,D.PERIOD,D.QTY
FROM MASTER M
CROSS APPLY
(
SELECT TOP 2 ID, PERIOD,QTY
FROM DETAILS D
WHERE M.ID=D.ID
ORDER BY CAST(PERIOD AS DATE)DESC
)D

- **[SQL FIDDLE][2]**

and forms the following result.

x------x---------x--------------x-------x
| Id | Name | PERIOD | QTY |
x------x---------x--------------x-------x
| 1 | A | 2014-01-13 | 10 |
| 1 | A | 2014-01-12 | 20 |
| 2 | B | 2014-01-08 | 40 |
| 2 | B | 2014-01-06 | 30 |
x------x---------x--------------x-------x

Here's how it works. The query inside `CROSS APPLY` can reference the outer table, where `INNER JOIN` cannot do this (it throws compile error). When finding the last two dates, joining is done inside `CROSS APPLY` i.e., `WHERE M.ID=D.ID`.

**2. When we need `INNER JOIN` functionality using functions.**

`CROSS APPLY` can be used as a replacement with `INNER JOIN` when we need to get result from `Master` table and a `function`.

SELECT M.ID,M.NAME,C.PERIOD,C.QTY
FROM MASTER M
CROSS APPLY dbo.FnGetQty(M.ID) C

And here is the function

CREATE FUNCTION FnGetQty
(
@Id INT
)
RETURNS TABLE
AS
RETURN
(
SELECT ID,PERIOD,QTY
FROM DETAILS
WHERE ID=@Id
)

- **[SQL FIDDLE][3]**

which generated the following result

x------x---------x--------------x-------x
| Id | Name | PERIOD | QTY |
x------x---------x--------------x-------x
| 1 | A | 2014-01-13 | 10 |
| 1 | A | 2014-01-11 | 15 |
| 1 | A | 2014-01-12 | 20 |
| 2 | B | 2014-01-06 | 30 |
| 2 | B | 2014-01-08 | 40 |
x------x---------x--------------x-------x



**ADDITIONAL ADVANTAGE OF CROSS APPLY**

`APPLY` can be used as a replacement for `UNPIVOT`. Either `CROSS APPLY` or `OUTER APPLY` can be used here, which are interchangeable.

Consider you have the below table(named `MYTABLE`).

x------x-------------x--------------x
| Id | FROMDATE | TODATE |
x------x-------------x--------------x
| 1 | 2014-01-11 | 2014-01-13 |
| 1 | 2014-02-23 | 2014-02-27 |
| 2 | 2014-05-06 | 2014-05-30 |
| 3 | NULL | NULL |
x------x-------------x--------------x

The query is below.

SELECT DISTINCT ID,DATES
FROM MYTABLE
CROSS APPLY(VALUES (FROMDATE),(TODATE))
COLUMNNAMES(DATES)

- **[SQL FIDDLE][4]**

which brings you the result

x------x-------------x
| Id | DATES |
x------x-------------x
| 1 | 2014-01-11 |
| 1 | 2014-01-13 |
| 1 | 2014-02-23 |
| 1 | 2014-02-27 |
| 2 | 2014-05-06 |
| 2 | 2014-05-30 |
| 3 | NULL |
x------x-------------x


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While most queries which employ CROSS APPLY can be rewritten using an INNER JOIN, CROSS APPLY can yield better execution plan and better performance, since it can limit the set being joined yet before the join occurs.

*Stolen from [Here][1]*


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Here's a brief tutorial that can be saved in a `.sql` file and executed in SSMS that I wrote for myself to quickly refresh my memory on how `CROSS APPLY` works and when to use it:

-- Here's the key to understanding CROSS APPLY: despite the totally different name, think of it as being like an advanced 'basic join'.
-- A 'basic join' gives the Cartesian product of the rows in the tables on both sides of the join: all rows on the left joined with all rows on the right.
-- The formal name of this join in SQL is a CROSS JOIN. You now start to understand why they named the operator CROSS APPLY.

-- Given the following (very) simple tables and data:
CREATE TABLE #TempStrings ([SomeString] [nvarchar](10) NOT NULL);
CREATE TABLE #TempNumbers ([SomeNumber] [int] NOT NULL);
CREATE TABLE #TempNumbers2 ([SomeNumber] [int] NOT NULL);
INSERT INTO #TempStrings VALUES ('111'); INSERT INTO #TempStrings VALUES ('222');
INSERT INTO #TempNumbers VALUES (111); INSERT INTO #TempNumbers VALUES (222);
INSERT INTO #TempNumbers2 VALUES (111); INSERT INTO #TempNumbers2 VALUES (222); INSERT INTO #TempNumbers2 VALUES (222);

-- Basic join is like CROSS APPLY; 2 rows on each side gives us an output of 4 rows, but 2 rows on the left and 0 on the right gives us an output of 0 rows:
SELECT
st.SomeString, nbr.SomeNumber
FROM -- Basic join ('CROSS JOIN')
#TempStrings st, #TempNumbers nbr
-- Note: this also works:
--#TempStrings st CROSS JOIN #TempNumbers nbr

-- Basic join can be used to achieve the functionality of INNER JOIN by first generating all row combinations and then whittling them down with a WHERE clause:
SELECT
st.SomeString, nbr.SomeNumber
FROM -- Basic join ('CROSS JOIN')
#TempStrings st, #TempNumbers nbr
WHERE
st.SomeString = nbr.SomeNumber

-- However, for increased readability, the SQL standard introduced the INNER JOIN ... ON syntax for increased clarity; it brings the columns that two tables are
-- being joined on next to the JOIN clause, rather than having them later on in the WHERE clause. When multiple tables are being joined together, this makes it
-- much easier to read which columns are being joined on which tables; but make no mistake, the following syntax is *semantically identical* to the above syntax:
SELECT
st.SomeString, nbr.SomeNumber
FROM -- Inner join
#TempStrings st INNER JOIN #TempNumbers nbr ON st.SomeString = nbr.SomeNumber

-- Because CROSS APPLY is generally used with a subquery, the subquery's WHERE clause will appear next to the join clause (CROSS APPLY), much like the aforementioned
-- 'ON' keyword appears next to the INNER JOIN clause. In this sense, then, CROSS APPLY combined with a subquery that has a WHERE clause is like an INNER JOIN with
-- an ON keyword, but more powerful because it can be used with subqueries (or table-valued functions, where said WHERE clause can be hidden inside the function).
SELECT
st.SomeString, nbr.SomeNumber
FROM
#TempStrings st CROSS APPLY (SELECT * FROM #TempNumbers tempNbr WHERE st.SomeString = tempNbr.SomeNumber) nbr

-- CROSS APPLY joins in the same way as a CROSS JOIN, but what is joined can be a subquery or table-valued function. You'll still get 0 rows of output if
-- there are 0 rows on either side, and in this sense it's like an INNER JOIN:
SELECT
st.SomeString, nbr.SomeNumber
FROM
#TempStrings st CROSS APPLY (SELECT * FROM #TempNumbers tempNbr WHERE 1 = 2) nbr

-- OUTER APPLY is like CROSS APPLY, except that if one side of the join has 0 rows, you'll get the values of the side that has rows, with NULL values for
-- the other side's columns. In this sense it's like a FULL OUTER JOIN:
SELECT
st.SomeString, nbr.SomeNumber
FROM
#TempStrings st OUTER APPLY (SELECT * FROM #TempNumbers tempNbr WHERE 1 = 2) nbr

-- One thing CROSS APPLY makes it easy to do is to use a subquery where you would usually have to use GROUP BY with aggregate functions in the SELECT list.
-- In the following example, we can get an aggregate of string values from a second table based on matching one of its columns with a value from the first
-- table - something that would have had to be done in the ON clause of the LEFT JOIN - but because we're now using a subquery thanks to CROSS APPLY, we
-- don't need to worry about GROUP BY in the main query and so we don't have to put all the SELECT values inside an aggregate function like MIN().
SELECT
st.SomeString, nbr.SomeNumbers
FROM
#TempStrings st CROSS APPLY (SELECT SomeNumbers = STRING_AGG(tempNbr.SomeNumber, ', ') FROM #TempNumbers2 tempNbr WHERE st.SomeString = tempNbr.SomeNumber) nbr
-- ^ First the subquery is whittled down with the WHERE clause, then the aggregate function is applied with no GROUP BY clause; this means all rows are
-- grouped into one, and the aggregate function aggregates them all, in this case building a comma-delimited string containing their values.

DROP TABLE #TempStrings;
DROP TABLE #TempNumbers;
DROP TABLE #TempNumbers2;

We use **CROSS APPLY** to update a table with **JSON** from another (update request) table -- joins won't work for this as we use OPENJSON, to read the content of the JSON, and OPENJSON is a "table-valued function".

I was going to put a simplified version of one of our UPDATE commands here as a example but, even simplified, it is rather large and overly complex for an example. So this much simplied "sketch" of just part of the command will have to suffice:

SELECT
r.UserRequestId,
j.xxxx AS xxxx,
FROM RequestTable as r WITH (NOLOCK)
CROSS APPLY
OPENJSON(r.JSON, '$.requesttype.recordtype')
WITH(
r.userrequestid nvarchar(50) '$.userrequestid',
j.xxx nvarchar(20) '$.xxx
)j
WHERE r.Id > @MaxRequestId
and ... etc. ....

Here is an article that explains it all, with their performance difference and usage over JOINS.

[SQL Server CROSS APPLY and OUTER APPLY over JOINS][1]


As suggested in this article, there is no performance difference between them for normal join operations (INNER AND CROSS).

[![enter image description here][2]][2]


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"SQL Server CROSS APPLY and OUTER APPLY"
[2]:

The usage difference arrives when you have to do a query like this:

CREATE FUNCTION dbo.fn_GetAllEmployeeOfADepartment(@DeptID AS INT)
RETURNS TABLE
AS
RETURN
(
SELECT * FROM Employee E
WHERE E.DepartmentID = @DeptID
)
GO

SELECT * FROM Department D
CROSS APPLY dbo.fn_GetAllEmployeeOfADepartment(D.DepartmentID)

That is, when you have to relate with function. This cannot be done using INNER JOIN, which would give you the error ***"The multi-part identifier "D.DepartmentID" could not be bound."*** Here the value is passed to the function as each row is read. Sounds cool to me. :)