During my work I get the chance reviewing lots of T-SQL Procedures and Views and I often see that the SQL joins are mis-used in them. When I enquire the developers regarding this, it’s evident that most of the time it has been the case that they don’t have the proper understanding what each JOIN exactly does or how it behaves, ultimately causing the SQL Procedure or the View to return an unexpected resultset. Therefore I thought of writing this blog post.
When we require to fetch details from multiple tables the JOIN caluse is there for the rescue. But in SQL Server there are various types of JOINs which will cater our requirement in different ways. So it’s very important to have a good understanding in these types of JOINs and their usage.
In SQL Server following types of JOINs available.
CROSS JOIN CROSS APPLY OUTER APPLY We will look into the afrementioned JOINs more closely. The scope of this article is to give a high-level idea on the aforementioned JOINs and the APPLY operator in SQL Server.
To illustrate the aforementioned JOINs I will use the following sample tables:
**Note: During the illustraion I will refer the table which is followed by the ‘FROM’ clause as the ‘Left Table’ and the table which is follwed by the JOIN clause as the ‘Right Table’.
![image]( "image")
This can be illustrated using a venn diagram as follows:
![image]( "image")
![image]( "image")
**Please note: We have sales reps having RepId’s 1,2,3,4, & 5. But in SalesDetails table we have sales details for RepId’s 1,2,4,6 &7. So when these tables are joined the RepId’s which resides on both tables, which are 1,2, and 4 will return the details, ultimately giving us the aforementioned result set.
![image]( "image")
This can be shown using a venn diagram as follows:
![image]( "image")
![image]( "image")
![image]( "image")
This can be shown using a venn diagram as follows:
![image]( "image")
![image]( "image")
![image]( "image")
This can be shown using a venn diagram as follows:
![image]( "image")
![image]( "image")
![image]( "image")
![image]( "image")
But when some condition is provided via the WHERE clause CROSS JOIN will behave like an INNER JOIN
![image]( "image")
**Note: In a CROSS JOIN it’s not possible to refer to a value in the Left table along with the right table. Example following code will result in an error.
For example, suppose we need to fetch sales rep details along with the maximum sale record which they have done. So the following query is not possible since it is returning an error due to the aforementioned reason.
Msg 4104, Level 16, State 1, Line 78
The multi-part identifier "RD.RepId" could not be bound.
The way to achieve this is by using an APPLY.
![image]( "image")
Noticed the above sample, you can see that it returned three records. But if you inspect closely, the SalesRep table consists with five Reps. But CROSS APPLY has only returned the maximum sales value if there’s a matching record in the table right side to the APPLY operator. (Similar to an INNER JOIN)
![image]( "image")
There are other capabilities which is possible using the APPLY. The following article explains these capabilites really well: http://bradsruminations.blogspot.sg/2011/04/t-sql-tuesday-017-it-slices-it-dices-it.html
Hope this will help you to understand the JOIN and the APPLY operator in SQL Server and where it can be used precisely.
When we require to fetch details from multiple tables the JOIN caluse is there for the rescue. But in SQL Server there are various types of JOINs which will cater our requirement in different ways. So it’s very important to have a good understanding in these types of JOINs and their usage.
In SQL Server following types of JOINs available.
- INNER JOIN
- OUTER JOIN
- LEFT OUTER JOIN
- RIGHT OUTER JOIN
- FULL OUTER JOIN
To illustrate the aforementioned JOINs I will use the following sample tables:
- SalesRep
- SalesDetails
- RepRating
- Settings
--== Create Tables ==--
CREATE TABLE RepDetails(
RepId INT
,RepName VARCHAR(30)
)
CREATE TABLE SalesDetails(
RepId INT
,SaleMonth VARCHAR(6)
,OrderNo VARCHAR(6)
,SaleValue MONEY
)
CREATE TABLE RepRating(
RepId INT
,Rate INT
,YearMonth VARCHAR(6)
)
CREATE TABLE Settings(
S_Id INT
,S_Desc VARCHAR(20)
,S_Value VARCHAR(20)
)
--== Populate Sample Data ==--
INSERT INTO RepDetails (
[RepId]
,[RepName]
) VALUES
(1,'Eugene Thomas')
,(2,'John Wheeler')
,(3,'Curtis Bailey')
,(4,'Jeffrey Garrett')
,(5,'Rosemarie Hubbard')
INSERT INTO SalesDetails (
[RepId]
,[SaleMonth]
,[OrderNo]
,[SaleValue]
)
VALUES
(7,'201607','XpyDy3',839)
,(1,'201607','NR0RTp',496)
,(4,'201607','4552T4',299)
,(6,'201607','GKhkyC',877)
,(4,'201606','iyK65Z',291)
,(6,'201606','NFCszW',446)
,(7,'201606','D238bN',135)
,(1,'201607','bERDXk',304)
,(7,'201608','nykZqB',935)
,(4,'201608','R7ea5v',352)
,(6,'201606','VVjIdo',407)
,(7,'201608','vtLT4z',977)
,(2,'201608','xnHTnO',416)
,(1,'201606','jFAJIm',674)
,(6,'201606','0Q011m',480)
INSERT INTO dbo.RepRating(
RepId
,Rate
,YearMonth
)
VALUES
(1,1,'201608')
,(3,2,'201608')
,(4,1,'201609')
,(2,2,'201609')
INSERT INTO dbo.Settings(
S_Id
,S_Desc
,S_Value
)
VALUES
(1,'LedgerMonth','201609')
,(2,'TaxRate','10%')
**Note: During the illustraion I will refer the table which is followed by the ‘FROM’ clause as the ‘Left Table’ and the table which is follwed by the JOIN clause as the ‘Right Table’.
INNER JOIN / JOIN
When we join two or more tables using an INNER JOIN, it will only return us the results when records can only be found on both left and right tables which will satisfy the condition we supply.
This can be illustrated using a venn diagram as follows:
SELECT *
FROM
dbo.RepDetails AS RD
JOIN dbo.SalesDetails AS SD
ON SD.RepId = RD.RepId
**Please note: We have sales reps having RepId’s 1,2,3,4, & 5. But in SalesDetails table we have sales details for RepId’s 1,2,4,6 &7. So when these tables are joined the RepId’s which resides on both tables, which are 1,2, and 4 will return the details, ultimately giving us the aforementioned result set.
LEFT OUTER JOIN / LEFT JOIN
In a LEFT OUTER JOIN, unlike the INNER JOIN, it will select all the records from the ‘Left’ table and based on the JOIN condition, it will select any matching records from the ‘Right’ table and return us the results. If there are no matching details on the ‘Right’ table, columns on related to those rows will return as ‘NULL’.
This can be shown using a venn diagram as follows:
SELECT *
FROM
dbo.RepDetails AS RD
LEFT JOIN dbo.SalesDetails AS SD
ON SD.RepId = RD.RepId
RIGHT OUTER JOIN / RIGHT JOIN
In a RIGHT OUTER JOIN, it will select all records from the ‘Right’ table and based on the JOIN condition it will select any matching records from the left table and return. If there aren’t any matching records on the left table it will return a ‘NULL’ value.
This can be shown using a venn diagram as follows:
SELECT *
FROM
dbo.SalesDetails AS SD
RIGHT JOIN dbo.RepDetails AS RD
ON SD.RepId = RD.RepId
FULL OUTER JOIN / FULL JOIN
FULL OUTER JOIN is kind of a mx of both LEFT & RIGHT OUTER JOINs. It will return all rows from both ‘Left’ and ‘Right’ tables based on the JOIN condition. When the details aren’t matched it will return a NULL value in those respective columns.
This can be shown using a venn diagram as follows:
SELECT *
FROM
dbo.RepDetails AS RD
FULL OUTER JOIN dbo.SalesDetails AS SD
ON SD.RepId = RD.RepId
CROSS JOIN
CROSS JOIN will return a result set which the number of rows equal to rows in ‘Left’ table multiplied by the number of rows in ‘Right’ table. Usually this behaviour is present when there’s no condition provided in the WHERE condition. So each row in the left table is joined to each row in the right table. Usuually this behaviour is called ‘Cartisian Product’
SELECT *
FROM
dbo.RepDetails AS RD
CROSS JOIN dbo.Settings AS S
But when some condition is provided via the WHERE clause CROSS JOIN will behave like an INNER JOIN
SELECT *
FROM
dbo.RepDetails AS RD
CROSS JOIN dbo.Settings AS S
WHERE
RD.RepId = S.S_Id
**Note: In a CROSS JOIN it’s not possible to refer to a value in the Left table along with the right table. Example following code will result in an error.
SELECT *
FROM
dbo.RepDetails AS RD
CROSS JOIN (SELECT * FROM dbo.Settings AS S WHERE S.S_Id = RD.RepId ) AS ST
CROSS APPLY behaves like an INNER JOIN and OUTER APPLY behaves like an OUTER JOIN. But the main differnce in APPLY compared to the JOIN is that the right side of the APPLY operator can reference columns in the table which is on the left side. This is not possible in a JOIN.
For example, suppose we need to fetch sales rep details along with the maximum sale record which they have done. So the following query is not possible since it is returning an error due to the aforementioned reason.
SELECTIt will result in an error:
*
FROM
dbo.RepDetails AS RD
JOIN(
SELECT TOP 1 *
FROM
dbo.SalesDetails AS SD
WHERE
RD.RepId = SD.RepId
ORDER BY
SD.SaleValue DESC
) AS SData
ON 1=1
Msg 4104, Level 16, State 1, Line 78
The multi-part identifier "RD.RepId" could not be bound.
The way to achieve this is by using an APPLY.
CROSS APPLY
Considering the above requirement, we can use a CROSS APPLY in order to achieve the aforementioned.SELECT
*
FROM
dbo.RepDetails AS RD
CROSS APPLY(
SELECT TOP 1 *
FROM
dbo.SalesDetails AS SD
WHERE
RD.RepId = SD.RepId
ORDER BY
SD.SaleValue DESC
) AS SData
Noticed the above sample, you can see that it returned three records. But if you inspect closely, the SalesRep table consists with five Reps. But CROSS APPLY has only returned the maximum sales value if there’s a matching record in the table right side to the APPLY operator. (Similar to an INNER JOIN)
OUTER APPLY
Using OUTER APPLY we can achieved a similar result like CROSS APPLY, but the difference is that even though there aren’t any matching records in the table right side to the APPLY operator, still it will return all the rows from the left side table, will NULL values for the columns in the right side table. We will consider the same query what we used in the above example, but changing the APPLY to an OUTER APPLY.SELECT
*
FROM
dbo.RepDetails AS RD
OUTER APPLY(
SELECT TOP 1 *
FROM
dbo.SalesDetails AS SD
WHERE
RD.RepId = SD.RepId
ORDER BY
SD.SaleValue DESC
) AS SData
There are other capabilities which is possible using the APPLY. The following article explains these capabilites really well: http://bradsruminations.blogspot.sg/2011/04/t-sql-tuesday-017-it-slices-it-dices-it.html
Hope this will help you to understand the JOIN and the APPLY operator in SQL Server and where it can be used precisely.
