SQL Left Join | SQL Left Outer Join

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: In 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 Left Outer join for better understanding.

From the above image you can understand easily that, 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:

Data present in the Department Table is:

SQL Left Join – Selecting All Columns

The following SQL 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 – Avoid Outer keyword

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

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

OUTPUT

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, Department Id for them in Employee table are NULLS so there is no matching records in 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

SQL Left Join – Selecting 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

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 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 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 don’t know which column you are asking it to retrieve.

To resolve these kind of issues you always have to use table name before the column name. The following 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 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 – Use Where Clause

The SQL Server 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 – Order By Clause

The SQL Server 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

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