How to write MySQL Left Join with an example using Command Prompt and Workbench?. The MySQL Left outer join is to return all the records (or rows) from the first table, and match rows from the right table.
The basic syntax of MySQL Left Join is as shown below:
SELECT Table1.Column(s), Table2.Column(s) FROM Table1 LEFT 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 OUTER JOIN Table2 ON Table1.Common_Column = Table2.Common_Column
It is optional for you to use the Outer Keyword because both are the same. Let us see the visual representation of the MySQL Left Outer join for better understanding.
From the above image, you can understand easily that the MySQL Left Outer join displays all the records present in Table A, and matching records from Table B.
NOTE: All the Unmatched rows from MySQL right table will fill with NULL Values.
To demonstrate this, we are going to use Employ, and Department tables present in our company Database. The data present in employ is:
Data present in the Department is:
MySQL Left Join Examples
The following are the list of ways we can use this one to combine two tables or get information (records) from two or more tables.
MySQL Left Join Select * Example
The following query will display all the columns present in the employ table and matching records from the Department table.
USE company; SELECT * FROM employ LEFT JOIN department ON employ.DeptID = department.DeptID;
If you observe the below screenshot, MySQL Left Join is displaying all the 15 records from the Employ. But for [DeptID], Department Name, and Standard_Salary, it is displaying NULL Values for the EmpID numbers 4, 5, 9, 11, and 15.
It is because DeptID for those records in the Employ is NULL. So there are no matching records in the right table.
NOTE: The DeptID column repeated twice, which might be annoying to the end-user. To avoid unwanted columns, I suggest you select individual column names. Please avoid SELECT * Statement.
MySQL Left Join select Few Columns Example
As we said before, please use the required columns after the SELECT Statement to avoid unwanted columns
USE company; SELECT First_Name, Last_Name, Education, Yearly_Income, Sales, DepartmentName, Standard_Salary FROM employ LEFT JOIN department ON employ.DeptID = department.DeptID;
Ambiguous Columns in Left Join Example
The above-specified MySQL Left join query will work entirely as long as the column names from both employ and Department are different like above. What happens if they had the same column names in both the tables? Well, with the above-specified approach, you will end up in a mess. Let us see how to resolve the issue.
Let me show you one practical example. As you can see, we are using the same MySQL left join query. But, we added DepID from the department as an additional column.
USE company; SELECT First_Name, Last_Name, Education, Yearly_Income, Sales, DeptID, DepartmentName, Standard_Salary FROM employ LEFT JOIN department ON employ.DeptID = department.DeptID;
As you can see, it is throwing an error: Ambiguous column DeptID. It is because, DeptID column is present in both tables, and the query doesn’t know which column you are asking it to display.
To resolve this kind of issue, always use the table name before the column name. The following query is using the ALIAS table name before the column names.
By this approach, we can inform the query that we are looking for DepID belonging to the department. We can simply write the above query as:
USE company; SELECT emp.First_Name, emp.Last_Name, emp.Education, emp.Yearly_Income, emp.Sales, dept.DeptID, dept.DepartmentName, dept.Standard_Salary FROM employ AS emp LEFT JOIN department AS dept ON emp.DeptID = dept.DeptID;
It is always a best practice to use the tablename before the Column name in MySQL Left Outer Join. For example, SELECT employ.First_Name
Left Join using Keyword
If the common column name in both the tables is the same, then you can use the MySQL Left Join with USING keyword. Since DeptID is the common column name in both the tables, the above query can also return as
USE company; SELECT emp.First_Name, emp.Last_Name, emp.Education, emp.Yearly_Income, emp.Sales, dept.DeptID, dept.DepartmentName, dept.Standard_Salary FROM employ AS emp LEFT JOIN department AS dept USING(DeptID);
MySQL Left Join Multiple Conditions
Until now, we are showing the = operator as the condition to the employ and department. But you can replace = with < (less than), > (greater than), or not equal to operators.
To connect those two tables, this example, we are using two conditions in this. The following query will display all the records from the employ and matching records from the department. Here matching records means, DeptID of both tables should match, and Standard Salary should be greater than 1000000
USE company; SELECT empl.First_Name, empl.Last_Name, empl.Education, empl.Yearly_Income, empl.Sales, dept.DepartmentName, dept.Standard_Salary FROM employ AS empl LEFT JOIN department AS dept ON empl.DeptID = dept.DeptID AND dept.Standard_Salary > 1000000;
I think you are confused about the result. Let me replace the greater than symbol with less than. It means, DeptID of both tables should match, and Standard Salary should be less than 1000000
MySQL Left Join Where Clause Example
In this example, we will show you how to use Where clause (to apply filter) along with this. I suggest you refer Where Clause article.
USE company; SELECT empl.First_Name, empl.Last_Name, empl.Education, empl.Yearly_Income, empl.Sales, dept.DepartmentName, dept.Standard_Salary FROM employ AS empl LEFT JOIN department AS dept ON empl.DeptID = dept.DeptID WHERE Sales > 20000;
Command prompt Example
Let me show you, How to write it in Command prompt. In this example, we will also show how to use Order By clause along with it. I suggest you refer to Order By article.
USE company; SELECT empl.First_Name, empl.Last_Name, empl.Education, depat.DepartmentName, depat.Standard_Salary, empl.Yearly_Income, empl.Sales FROM employ AS empl LEFT JOIN department AS depat ON empl.DeptID = depat.DeptID ORDER BY First_Name, Last_Name;