SQL Left Join | SQL Left Outer Join

The SQL Left Join is a SQL Join Type used to return all the rows or records present in the Left table and matching rows from the right table. Let us see the visual representation of the SQL Server Left Outer join for better understanding.

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

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

SQL LEFT JOIN Syntax

The syntax of the SQL Server Left Join is

-- 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 Left Outer join can also call as Left Join. So it is optional to use the Outer Keyword.

For this Sql Server Left Join example, We are going to use two tables in our [SQL Server Tutorials] Database.

Data present in the Employee Table is:

The SQL Server Data present in the Department Table is:

SQL Left Join Select All Columns

The following 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

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

If you observe the above screenshot, We have 15 records in the Employee table. And the Left Outer Join is displaying 15 records, but for [DepartID], id, [Department Name], it is displaying NULL Values for ID numbers 10, 11, 14, and 15. It is because the Department Id for them in the Employee table is NULLS, so there are 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

SQL Left Join Select Few Columns

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

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

OUTPUT

The above query work as long as the column names from both tables are different like above. If we have the identical Column names in both the tables, left join will throw an error. Let us see how to fix the issue.

Before we get into the answer, let me present you with one practical example. As you can see, we are using the above 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]

As you can see, the left join is throwing an error: Ambiguous column name id. It is because the id column is present in both the tables. And SQL Server doesn’t understand which column you are requesting it to retrieve.

To resolve this sort of issue, forever use the table name before the column name. The following left Outer Join query is using the ALIAS table name before the column names. By this method, we can inform the SQL Server that we are looking for id column relating to the department table.

We can rewrite 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

Left Join Where Clause

The SQL Server Left Join also permits us to use Where Clause to limit the number of rows returned by the Left Outer Join. In this example, we use 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

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