Starting with SQL Server 2005, you can use the APPLY operator in a Transact-SQL query to join a table to a table-valued function so the function is evoked for each row returned from the table. For example, you might create a query that returns a list of employees from the Employee table. For each employee, you also want to return a list of the individual job positions that person has held in the company.

You can create a function that retrieves the employees’ positions and then evoke that function for each row returned from the Employee table. Your result set will then include a row for each position that an employee has held. For instance, if John has worked as a salesman and a regional supervisor, the result set will include two rows for John, one for each position. Any data returned from the Employee table will be repeated for each row, but the data returned by the function will be specific to each row in the function’s results.

The APPLY operator can take one of two forms: CROSS APPLY or OUTER APPLY. The CROSS APPLY operator returns rows from the primary (outer) table only if the table-value function produces a result set. That means, in the example above, an employee would be included in the returned data only if that employee has held a specific position within the company. The OUTER APPLY form, on the other hand, returns all rows from the outer table, even if the function produces no results. So an employee would be listed even if that employee held no specific position.

In this article, I demonstrate how to work with both forms of the APPLY operator. The examples I show you were created on a local instance of SQL Server 2008 and the AdventureWorks2008 sample database. If you want to run these examples against the AdventureWorks database on an instance of either SQL Server 2005 or 2008, you must change references to the BusinessEntityID column to SalesPersonID column, where appropriate. In addition, you’ll find that your results might vary slightly from those shown here because the values between the databases are slightly different, particularly primary key values.

Using the CROSS APPLY Operator

As I mentioned, CROSS APPLY returns only those rows in the outer table for which the table value function returns data. Let’s look at an example to demonstrate how this works. Fist, we’ll create a function that returns the top three sales generated by a salesperson, as those sales appear in the Sales.SalesOrderHeader table in the AdventureWorks2008 database:

As you can see, the fn_sales function takes one parameter, @SalesPersonID, which is configured with the int data type. The function returns the three highest sales for the specified salesperson. Note that this is a table-valued function, which means that it returns the entire result set generated by the SELECT statement. For the fn_sales function, the result set includes the SalesPersonID and SalesAmount columns.

After you create your function, you can test it by running a SELECT statement that retrieves data from the function. For example, the following SELECT statement returns the SalesAmount column for salesperson ID 285:

The following table shows the results returned by the statement. As you would expect, three rows have been returned-the three highest sales for this salesperson.

SalesAmount

85652.33

45338.76

36317.54

After you’ve verified that the function is returning the correct results, you can use the function within a statement that includes the CROSS APPLY operator. In the following example, I retrieve data from the Sales.vSalesPerson view and join it to the fn_sales function:

As you can see, I use the CROSS APPLY operator in the FROM clause by first specifying the outer table (vSalesPerson), then the CROSS APPLY operator, and finally the fn_sales function. Notice that I pass in the BusinessEntityID value as the function’s parameter. This value is based on the BusinessEntityID value as it appears in the current row that is being returned from the outer table.

Also notice that the columns in the SELECT list reference the source table and function as they would if I were joining two tables. I assign an alias to the table (sp) and one to the function (fn) and then reference the columns accordingly. For example, because the SalesAmount column in the SELECT list is returned by the function, I qualify the column name as fn.SalesAmount.

The following table shows part of the results returned by the SELECT statement above. Notice that each salesperson is listed three times, once for each result returned by the fn_sales function.

FullName	SalesAmount
Syed Abbas	85652.33
Syed Abbas	45338.76
Syed Abbas	36317.54
Amy Alberts	98405.08
Amy Alberts	96243.80
Amy Alberts	95193.67
Pamela Ansman-Wolfe	125254.49
Pamela Ansman-Wolfe	125144.01
Pamela Ansman-Wolfe	118284.78
Michael Blythe	198628.31
Michael Blythe	142942.01
Michael Blythe	139659.67
David Campbell	149897.36
David Campbell	136046.44
David Campbell	125068.34
Jillian Carson	162629.75
Jillian Carson	154912.07
Jillian Carson	144355.88
Shu Ito	247913.91
Shu Ito	227737.72
Shu Ito	189198.62
Stephen Jiang	149861.07
Stephen Jiang	114361.94
Stephen Jiang	112733.70
Tete Mensa-Annan	140734.49
Tete Mensa-Annan	137108.39
Tete Mensa-Annan	115068.64

That’s all there is to using the CROSS APPLY operator. Not let’s look at another example that uses the operator with a common table expression (CTE).

Using the CROSS APPLY Operator with a CTE

To demonstrate how you can use the APPLY operator with a CTE, I created the following function, which returns the product model associated with the specified product:

The function joins the Production.Product and Production.ProductModel tables to return the product name and model name for the specified product ID. I then used the following SELECT statement to verify that the function works as I expect:

The statement returns the product name and model for product 707 (shown in the following table). As you can see, the product Sport-100 Helmet, Red is associated with the model Sport-100.

ProductName	ModelName
Sport-100 Helmet, Red	Sport-100

You can then use the CROSS APPLY operator to join a CTE to the function, as shown in the following example:

Notice that I first define a CTE named ProductSales. The CTE returns the total sales for each product as they appear in the Sales.SalesOrderDetail table. I then use the CTE in the main SELECT statement, along with the fn_products function. Notice that I use the CROSS APPLY operator in the FROM clause to join the CTE to the function, as I would use the operator to join a table or view to the function. The following table shows part of the results returned by the SELECT statement.

ProductID	TotalSales	ProductModel
707	157772.394392	Sport-100
708	160869.517836	Sport-100
709	6060.388200	Mountain Bike Socks
710	513.000000	Mountain Bike Socks
711	165406.617049	Sport-100
712	51229.445623	Cycling Cap
713	21445.710000	Long-Sleeve Logo Jersey
714	115249.214976	Long-Sleeve Logo Jersey
715	198754.975360	Long-Sleeve Logo Jersey
716	95611.197080	Long-Sleeve Logo Jersey
717	394255.572400	HL Road Frame
718	395182.699300	HL Road Frame
719	89872.173600	HL Road Frame
722	177635.904000	LL Road Frame
723	24844.692200	LL Road Frame
725	194692.599104	LL Road Frame
726	132125.252200	LL Road Frame
727	20104.443400	LL Road Frame
729	195933.409400	LL Road Frame
730	137213.485128	LL Road Frame
732	89224.500000	ML Road Frame
733	32120.820000	ML Road Frame
736	45164.684600	LL Road Frame
738	299595.522966	LL Road Frame
739	269874.009600	HL Mountain Frame
741	141635.100000	HL Mountain Frame
742	499556.572400	HL Mountain Frame
743	901590.233600	HL Mountain Frame
744	13765.920000	HL Mountain Frame
745	106078.560000	HL Mountain Frame

One thing you might notice about the results shown here, compared to the results shown in the preceding example, is that the function returns only one row for each product. Because a product is associated with only one product model in the SalesOrderDetail table, there will never be more than one row per product.

Using the OUTER APPLY Operator

In the examples above, the CROSS APPLY operator returns the rows in the outer table for which the table value function returns data. As it turns out, the functions used in both examples return data for all rows in the outer tables. However, in some cases, the function will not return data for a specific row. If you still want the row from the outer table to be included in the result set, you should use the OUTER APPLY operator.

The OUTER APPLY operator returns all rows from the outer table, whether or not the function returns data for a specific row. You use the OUTER APPLY operator just as you would CROSS APPLY; the difference is in the results. Let’s look at an example that demonstrates how this works.

In the following statement, I create a function that returns data about product inventory:

This statement, just like the CREATE FUNCTION statements used in the preceding examples, creates a table-valued function. The function returns three columns: ProductID, LocationID, and Quantity. I verified the function by using the following SELECT statement:

In this case, I specified the product ID of 915 as the function’s parameter. The following table shows the results returned by the statement.

LocationID	Quantity
6	161
50	83
60	158

As you can see, product inventory exists in three locations, with different quantities at each location. Once the function has been verified, you can use it with a CROSS APPLY operator, as shown in the following statement:

Notice that this statement is similar to the previous examples. I’ve simply used the CROSS APPLY operator to join the Product table to the fn_inventory function. The following table shows part of the data returned by the statement. As the results indicate, there are multiple rows for each product, one for each quantity.

ProductID	Name	Quantity
915	ML Touring Seat/Saddle	161
915	ML Touring Seat/Saddle	83
915	ML Touring Seat/Saddle	158
916	HL Touring Seat/Saddle	425
916	HL Touring Seat/Saddle	288
916	HL Touring Seat/Saddle	276
921	Mountain Tire Tube	286
921	Mountain Tire Tube	243
922	Road Tire Tube	264
922	Road Tire Tube	241
923	Touring Tire Tube	262
923	Touring Tire Tube	240
928	LL Mountain Tire	240
928	LL Mountain Tire	369
929	ML Mountain Tire	385
929	ML Mountain Tire	284
930	HL Mountain Tire	267
930	HL Mountain Tire	232

The results shown in the table are those returned for product IDs 915 through 930. As you would expect, each row includes data returned from both the table and the function. Now let’s look at an example that uses OUTER APPLY, rather than CROSS APPLY:

This statement is exactly like the previous one, except for the APPLY operator. However, the results are slightly different, as shown in the following table:

ProductID	Name	Quantity
915	ML Touring Seat/Saddle	161
915	ML Touring Seat/Saddle	83
915	ML Touring Seat/Saddle	158
916	HL Touring Seat/Saddle	425
916	HL Touring Seat/Saddle	288
916	HL Touring Seat/Saddle	276
917	LL Mountain Frame – Silver, 42	NULL
918	LL Mountain Frame – Silver, 44	NULL
919	LL Mountain Frame – Silver, 48	NULL
920	LL Mountain Frame – Silver, 52	NULL
921	Mountain Tire Tube	286
921	Mountain Tire Tube	243
922	Road Tire Tube	264
922	Road Tire Tube	241
923	Touring Tire Tube	262
923	Touring Tire Tube	240
924	LL Mountain Frame – Black, 42	NULL
925	LL Mountain Frame – Black, 44	NULL
926	LL Mountain Frame – Black, 48	NULL
927	LL Mountain Frame – Black, 52	NULL
928	LL Mountain Tire	240
928	LL Mountain Tire	369
929	ML Mountain Tire	385
929	ML Mountain Tire	284
930	HL Mountain Tire	267
930	HL Mountain Tire	232

The result set now includes rows for products 917 through 920 and products 924 through 927, which were not included in the previous results. Notice that the Quantity column shows a NULL value for each of the new rows. By using the OUTER APPLY operator, we’re able to return all rows from the outer table, whether or not the function returns any rows.

As you can see, the APPLY operator can be a useful tool when you want to evoke a table-valued function for each row returned by a table expression (the outer table). You simply use the operator to join the outer table to the function. If you want to include only those rows from the outer table for which the function returns data, use the CROSS APPLY operator. If you want to return all rows from the outer table, regardless of whether or not the function returns data for a row, use the OUTER APPLY operator. For more details about either form of the APPLY operator and to see additional examples, check out the topic “Using APPLY” in SQL Server Books Online.