Introduction
A derived table is a select statement inside parenthesis,
with an alias, used as a table in a join.
A simple example is:
SELECT a.LastName, b.State
FROM UserTable a INNER JOIN
(SELECT UserID, State FROM AddressTable)
b
ON a.UserID = b.UserID
The table with the alias b is the derived table. Of course, in the above example you could
just as easily do the join without the derived table. All code in this article was tested on SQL
Server 7.0 and SQL Server 2000.
Derived Table Example 1
I have found two cases where a derived table can be useful
First we need to set up a few test tables. You can copy and paste the following code
into Query Analyzer:
CREATE TABLE #Components (UserID
int, ComponentName varchar(20),
SerialNumber varchar(10))
CREATE TABLE #Users (UserID int, LocationID int, UserName varchar(20))
CREATE TABLE #Locations (LocationID
int, State char(2))
INSERT INTO #Locations (LocationID,
State)
SELECT 1, 'VA'
UNION
SELECT 2, 'MD'
INSERT INTO #Users (UserID, LocationID, UserName)
SELECT 1, 1, 'Robert Marda'
UNION
SELECT 2, 1, 'Arthur Landers'
UNION
SELECT 3, 2, 'Flecha Ardiente'
UNION
SELECT 4, 2, 'Lia Bakker'
INSERT INTO #Components (UserID, ComponentName, SerialNumber)
SELECT 1, 'Computer', 'A003B2'
UNION
SELECT 1, 'Computer', 'A003B5'
UNION
SELECT 2, 'Computer', 'A003B7'
UNION
SELECT 2, 'Laptop', 'CG15R4D'
UNION
SELECT 3, 'Computer', 'A003B10'
UNION
SELECT 3, 'Laptop', 'CG15R1Z'
UNION
SELECT 3, 'Computer', 'A003B0'
UNION
SELECT 4, 'Laptop', 'CG15R4W'
UNION
SELECT 4, 'Computer', 'A003B6'
UNION
SELECT 4, 'Laptop', 'CG15R3E'
Now for our first result set lets
suppose we need to know how many users are at each location and how many
components are at each location. Here is
a query I have seen developers use to try and get the desired results:
SELECT DISTINCT l.State, COUNT(u.LocationID) AS UserCount, COUNT(c.UserID) AS ComponentCount
FROM #Locations l
INNER JOIN #Users u ON l.LocationID
= u.LocationID
INNER JOIN #Components c ON c.UserID
= u.UserID
GROUP BY l.State
This query will return the following results:
State UserCount ComponentCount
----- ----------- --------------
MD 6 6
VA 4 4
(2 row(s) affected)
The query returns the correct number of components but not
the correct number of users. One way to
get the desired results without changing the database structure is to use a
derived table like this:
SELECT DISTINCT l.State, COUNT(u.LocationID) AS UserCount, c.ComponentCount
FROM #Locations l
INNER JOIN #Users u ON l.LocationID
= u.LocationID
INNER JOIN (SELECT u.LocationID, COUNT(c.ComponentName) AS ComponentCount
FROM #Components c
INNER JOIN #Users u ON c.UserID
= u.UserID
GROUP
BY u.LocationID) c ON c.LocationID
= l.LocationID
GROUP BY l.State,c.ComponentCount
ORDER BY l.State
Executing this query returns the following results:
State UserCount ComponentCount
----- ----------- --------------
MD 2 6
VA 2 4
(2 row(s) affected)
The derived table is everything in the parenthesis and has
the alias name c. To get the desired
results we simply moved one of the count functions into the derived table and
the results came back as needed.
Derived Table Example 2
This next example shows a case where a simple join will not work. This occurs when you need to use the key word distinct and you are using the case statement in the order by clause. Using the same
tables and test data previously created lets say we need to know what
components are at each location. We want
to be able to sort by the state or the component name by setting a
variable. The following query could be
used:
DECLARE @orderby int
SET @orderby = 1
SELECT
l.State, ComponentName
FROM #Locations l
INNER JOIN #Users u ON l.LocationID
= u.LocationID
INNER JOIN #Components c ON c.UserID
= u.UserID
ORDER BY CASE @orderby
WHEN 1 THEN l.State
WHEN 2 THEN ComponentName
END
Here are the results:
State ComponentName
----- --------------------
MD Laptop
MD Laptop
MD Computer
MD Computer
MD Laptop
MD Computer
VA Computer
VA Computer
VA Computer
VA Laptop
(10 row(s) affected)
Now we need to eliminate the duplicates and so we add the
distinct key word to the query:
DECLARE @orderby int
SET @orderby = 1
SELECT DISTINCT
l.State, ComponentName
FROM #Locations l
INNER JOIN #Users u ON l.LocationID
= u.LocationID
INNER JOIN #Components c ON c.UserID
= u.UserID
ORDER BY CASE @orderby
WHEN 1 THEN l.State
WHEN 2 THEN ComponentName
END
However, when we execute the modified query we get the
following error:
Server: Msg 145, Level 15, State
1, Line 5
ORDER BY items must appear in the select list if SELECT
DISTINCT is specified.
I will not claim to understand why the error is given since
as far as I can see I have specified both columns in the order by and the
select list. Just adding the keyword
distinct breaks the query. The way I
found to get around this problem is to use a derived table and put the SELECT
DISTINCT in the derived table. The
modified query looks like this:
DECLARE @orderby int
SET @orderby = 1
SELECT --DISTINCT
l.State, ComponentName
FROM #Locations l
INNER JOIN
(SELECT DISTINCT LocationID, ComponentName FROM #Users u
INNER JOIN #Components c ON c.UserID
= u.UserID) derived ON l.LocationID
= derived.LocationID
ORDER BY CASE @orderby
WHEN 1 THEN l.State
WHEN 2 THEN ComponentName
END
This query gives us the following result set:
State ComponentName
----- --------------------
MD Computer
MD Laptop
VA Computer
VA Laptop
(4 row(s) affected)
In the above query we converted a join between the table
#Users and #Components into the derived table and moved the distinct keyword to
the derived table and were able to get the desired results.
Conclusions
I use derived tables whenever convenient. It is often easier than creating a temp table and usually works better. The above two examples show cases when it can be useful, perhaps required to use something other than a simple join and derived tables work well in both the examples I have given.
