Understanding JOINs in SQL Server

During my work, I get a chance to review lots of T-SQL Procedures and Views and I often see that the SQL joins are misused in them. When I ask developers regarding this, it’s evident that most of the time it has been the case that they don’t have proper understanding of what each JOIN exactly does or how it behaves, ultimately causing the SQL Procedure or the View to return an unexpected result set. Therefore, I thought of writing this blog post.

When we require fetching details from multiple tables, the JOIN clause is there to the rescue. But in SQL Server, there are various types of JOINs which will cater to our requirement in different ways. So it’s very important to have a good understanding of these types of JOINs and their usage.

In SQL Server, the following types of JOINs are available:

• INNER JOIN
• OUTER JOIN
  • LEFT OUTER JOIN
  • RIGHT OUTER JOIN
  • FULL OUTER JOIN
• CROSS JOIN
• CROSS APPLY
• OUTER APPLY

We will look into the aforementioned 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:

• SalesRep
• SalesDetails
• RepRating
• Settings

We consider a case where we have 5 Sales Reps and the details will be saved in ‘RepDetails’ table and the sales transactions which they have done is recorded under ‘SalesDetails’ table. In the SalesDetails table, we have included few transactions which we don’t have a matching Sales Rep. Similarly, in the RepDetails table, there are a couple of sales reps for which we don’t have any sales information.

--== 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 illustration, I will refer to the table which is followed by the ‘FROM’ clause as the ‘Left Table’ and the table which is followed 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 RepIds 1,2,3,4, & 5. But in SalesDetails table, we have sales details for RepIds 1,2,4,6 &7. So when these tables are joined the RepIds which reside 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 mix 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 with 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. Usually, this behaviour is called ‘Cartesian 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: the 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 difference 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.

SELECT 
 *
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

It will result in an error:

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 on 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 that 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 are possible using the APPLY. This article explains these capabilities really well.

Hope this will help you to understand the JOIN and the APPLY operator in SQL Server and where it can be used precisely.