You must be wondering what is this correlated join in SQL Server? Let
me clarify it’s not a type of join like INNER or OUTER, it is a kind
of usage or play around with joins. Some call it derived join
too. Anyway it really does not matter what it is called. The concept is
important to understand here. Different types of JOINs in SQL Server
Couple of months ago, I was interviewing a guy. My first question was, find the
top 3 business by Amount from Business table, he was kind of happy
because he thought it’s a very silly and common question.
I saw that happiness and tweaked the question a little bit, I asked him to
get me the same out put but by doing a LEFT JOIN with Employee then
INNER JOIN between Employee and Location tables.
Let’s do the practical. We have 3 tables, named Employee, Location and
Business. In Business table, we have the business data. Let’s find the top 3
businesses by Amount made so far by our employees using the joins mentioned
above.
Business:
BusinessID
|
BusinessName
|
EmpID
|
Amount
|
1
|
92 Per-cents %% team
|
7
|
4750
|
2
|
K-Land fund trust
|
2
|
16750
|
3
|
Robert@BigStarBank.com
|
7
|
18100
|
5
|
BIG 6’s Foundation%
|
4
|
22090
|
6
|
TALTA_Kishan International
|
3
|
18100
|
7
|
Ben@MoreTechnology.com
|
10
|
41000
|
8
|
www.@-Last-U-Can-Help.com
|
7
|
25000
|
9
|
Thank you @.com
|
11
|
21500
|
10
|
Just Mom
|
5
|
9900
|
11
|
Big Giver Tom
|
7
|
19000
|
12
|
Mega Mercy
|
9
|
55000
|
13
|
Hope Reaches
|
7
|
29000
|
14
|
Everyone Wins
|
4
|
12500
|
Employee:
EmpID
|
LastName
|
FirstName
|
LocationID
|
ManagerID
|
2
|
Brown
|
Barry
|
1
|
11
|
3
|
Osako
|
Lee
|
2
|
11
|
4
|
Kinnison
|
Dave
|
1
|
11
|
5
|
Bender
|
Eric
|
1
|
11
|
7
|
Lonning
|
David
|
NULL
|
11
|
9
|
Newton
|
James
|
2
|
3
|
10
|
O’Haire
|
Terry
|
2
|
3
|
11
|
Smith
|
Sally
|
1
|
NULL
|
Location
LocationID
|
street
|
city
|
state
|
1
|
545 Pike
|
Seattle
|
WA
|
2
|
222 Second AVE
|
Boston
|
MA
|
4
|
444 Ruby ST
|
Spokane
|
WA
|
5
|
1595 Main
|
Philadelphia
|
PA
|
6
|
915 Wallaby Drive
|
Sydney
|
NULL
|
TOP 3 Business by Amount, our desired output:
BusinessName
|
Amount
|
EmpID
|
Mega Mercy
|
55000
|
9
|
Ben@MoreTechnology.com
|
41000
|
10
|
Hope Reaches
|
29000
|
7
|
To get the above result, we can simply write a SELECT
TOP(3) statement on Business table and then ORDER BY Amount DESC like
shown below.
SELECT TOP (3) BusinessName, Amount from [dbo].[Business]
|
|
ORDER BY Amount DESC
|
|
Remember, I had tweaked the question and asked him to write the query by
doing LEFT JOIN between Business and Employee tables then INNER JOIN between
Employee and Location tables. Usually someone will write a query like below.
--Wrong
|
|
SELECT TOP (3) b.BusinessName, b.Amount FROM Business b
|
|
LEFT JOIN Employee e
|
|
ON e.EmpID =
b.EmpID
|
|
INNER JOIN Location l
|
|
ON l.LocationID =
e.LocationID
|
|
ORDER BY b.Amount DESC
|
Output: (Wrong)
BusinessName
|
Amount
|
EmpID
|
Mega Mercy
|
55000
|
9
|
Ben@MoreTechnology.com
|
41000
|
10
|
BIG 6’s Foundation%
|
22090
|
4
|
We know BIG 6’s Foundation% (22090) is not our 3rd highest
business. The reason Hope Reaches(29000) did not come in the
result set because EmpID 7 owns that business and EmpID
7 has no relative data in the location table, LocationID is
NULL for EmpID 7 in the Employee table.
Note: In the above query we are fine up to LEFT JOIN between
Business and Employee tables but the INNER JOIN with Location phrased out the
whole result set at the end with only matching records between Employee and
Location tables. Our EmpID 7 is the owner of the 3rd highest business
unfortunately he works from home so his business is filtered out and
the next highest business is displayed.
Now answering the question.. There are number of ways to get the desired
result set but probably the most natural is to use an interesting capability of
joins.
--Right
|
|
SELECT TOP (3) b.BusinessName, b.Amount, b.EmpID FROM Business b
|
|
LEFT JOIN (
|
|
Employee e
|
|
INNER JOIN Location l
|
|
ON l.LocationID =
e.LocationID
|
|
)
|
|
ON e.EmpID =
b.EmpID
|
|
ORDER BY b.Amount DESC
|
Output: (Right)
BusinessName
|
Amount
|
EmpID
|
Mega Mercy
|
55000
|
9
|
Ben@MoreTechnology.com
|
41000
|
10
|
Hope Reaches
|
29000
|
7
|
In the above query, we isolated the EMPLOYEE and LOCATION join
within parenthesis to their own pendent logical phase. Which is why the
LEFT JOIN is not filtered out by the location anymore.
