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SQL LEFT JOIN

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The SQL Left Join is a Join used to return all the records (or rows) present in the Left table and matching rows from the right table.

NOTE: All the Unmatched rows from the right table will be filled with NULL Values.

SQL LEFT JOIN Syntax

The basic syntax of the Left Join in SQL Server is as follows:

-- SQL Server LEFT JOIN Syntax
SELECT Table1.Column(s), Table2.Column(s),
FROM Table1
 LEFT OUTER JOIN
     Table2 ON
   Table1.Common_Column = Table2.Common_Column

--OR We can Simply Write it as
SELECT Table1. Column(s), Table2. Column(s),
FROM Table1
 LEFT JOIN
     Table2 ON
   Table1.Common_Column = Table2.Common_Column

TIP: The SQL Server Left Outer join can also be called as Left Join. So it is optional to use the Outer Keyword.

Let us see the visual representation of the SQL Left Outer join for better understanding.

SQL LEFT OUTER JOIN

From the above image, you can understand easily that, Sql Server Left Outer join displays all the records present in Table 1 and matching records from Table 2

In this article, we will show you, How to write Left Join in SQL Server. For this, We are going to use two tables (Employee and Department) present in our [SQL TEST] Database.

Data present in the Employee Table is:

SQL LEFT JOIN

Data present in the Department Table is:

SQL LEFT JOIN

SQL Left Join Select All Columns

The following Sql Server Left Outer Join Query will display all the columns present in Employees, and matching records from Department tables

-- SQL Server LEFT JOIN Example
SELECT *
FROM [Employee]
 LEFT OUTER JOIN
     [Department] ON
 [Employee].[DepartID] = [Department].[id]

OUTPUT

SQL LEFT JOIN 1

SQL Left Join – Avoid Outer keyword

As we said before, it is optional to use an Outer keyword. Let me remove the Outer keyword, and work will LEFT JOIN

-- SQL Server LEFT JOIN Example
SELECT *
FROM [Employee]
 LEFT JOIN
     [Department] ON
 [Employee].[DepartID] = [Department].[id]

OUTPUT

SQL LEFT JOIN Example 1

If you observe the above screenshot, We have 15 records in Employee table, Left Outer Join is displaying 15 records but for [DepartID], id, [Department Name] it is displaying NULL Values for the ID number 10, 11, 14 and 15 . This is because the Department Id for them in Employee table are NULLS so there is no matching records in the right table.

NOTE: The [Department ID] column is repeated twice, which is annoying to the user. By selecting individual column names we can avoid unwanted columns so, Please avoid SELECT * Statements in Left Join in Sql Server

SQL Left Join Select Few Columns

Please place the required columns after the SELECT Statement to avoid unwanted columns

-- SQL Server LEFT JOIN Example
SELECT [FirstName]
      ,[LastName]
      ,[DepartmentName]
FROM [Employee]
  LEFT JOIN
     [Department] ON
  [Employee].[DepartID] = [Department].[id]

OUTPUT

SQL LEFT JOIN 2

Above query will perfectly work as long as the column names from both tables (Employee and Department) are different like above. What happens if we have the same Column names in both the tables? Well, you will end up in a mess. Let us see how to resolve the issue.

Before we get into the solution, let me show you one practical example. As you can see, we are using the above Sql Server Left Join query but we added id from department table as an additional column.

-- SQL Server LEFT JOIN Example
SELECT [FirstName]
      ,[LastName]
      ,id
      ,[DepartmentName]
FROM [Employee]
LEFT OUTER JOIN
     [Department] ON
           [Employee].[DepartID] = [Department].[id]

You can see from the below screenshot, it is throwing an error: Ambiguous column name id. This is because, id column is present in both Employee, and department table. And SQL Server doesn’t know which column you are asking it to retrieve.

SQL LEFT JOIN Example 2

To resolve these kinds of issues you always have to use table name before the column name. The following Sql Server left Outer Join query is using ALIAS table name before the column names. By this approach, we can inform the SQL Server, that we are looking for id column belonging to the department table.

We can simply write the above query as:

-- SQL Server LEFT JOIN Example
SELECT Emp.[FirstName] AS [First Name]
      ,Emp.[LastName] AS [Last Name]
	  ,Dept.id 
      ,Dept.[DepartmentName] AS [Department Name]
FROM [Employee] AS Emp
LEFT JOIN
     [Department] AS Dept ON
          Emp.[DepartID] = Dept.[id]

OUTPUT

SQL LEFT JOIN Example 3

SQL Left Join Where Clause

The SQL Server Left Join also allows us to use Where Clause to restrict the number of rows returned by the Left Outer Join. In this example, we will use that WHERE Clause along with the Left Outer Join.

-- SQL Server LEFT JOIN Example
SELECT Emp.[FirstName] AS [First Name]
      ,Emp.[LastName] AS [Last Name]
      ,Dept.[DepartmentName] AS [Department Name]
FROM [Employee] AS Emp
LEFT JOIN
     [Department] AS Dept ON
          Emp.[DepartID] = Dept.[id]
WHERE Dept.[DepartmentName] IS NOT NULL

OUTPUT

SQL LEFT JOIN Example 4

SQL Left Join Order By Clause

The SQL Server Left Outer Join allows us to use Order By Clause in Left Outer Join to rearrange the order of the records.

-- SQL Server LEFT JOIN Example
SELECT Emp.[FirstName] AS [First Name]
      ,Emp.[LastName] AS [Last Name]
      ,Dept.[DepartmentName] AS [Department Name]
FROM [Employee] AS Emp
 LEFT JOIN
     [Department] AS Dept ON
    Emp.[DepartID] = Dept.[id]
ORDER BY [DepartmentName] ASC

OUTPUT

SQL LEFT JOIN 3

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