When it comes to database management systems, the concept of joins is crucial. A join is used to combine data from two or more tables in a database. There are various types of joins, but the most commonly known are the Left Join and Left Outer Join. In this article, we will explore the differences between Left Join and Left Outer Join and compare them.
Introduction to Left Join and Left Outer Join
First, let’s define what Left Join and Left Outer Join are:
– Left Join: A Left Join is a type of join that returns all rows from the left table specified in the join clause and only matching rows from the right table.
– Left Outer Join: A Left Outer Join is another type of join that returns all rows from the left table specified in the join clause and all matching rows from the right table. If there are no matching rows in the right table, the result will still include the rows from the left table.
Both joins are used to combine data from two or more tables in a database, but the difference lies in how they handle non-matching rows.
Difference between Left Join and Left Outer Join
The main difference between Left Join and Left Outer Join is how they handle non-matching rows. Let’s look closer at each join and what happens when there are non-matching rows in the tables:
Left Join: When there are non-matching rows in the right table, the result will still include all rows from the left table. The columns from the right table will be filled with NULL values.
Let’s say we have two tables, Customers and Orders. The Customers table has the following data:
| CustomerID | CustomerName |
|————|————–|
| 1 | John |
| 2 | Jane |
The Orders table has the following data:
| OrderID | CustomerID |
|———|————|
| 1 | 1 |
| 2 | 3 |
If we perform a Left Join on the Customers table and Orders table, we will get the following result:
| CustomerID | CustomerName | OrderID | CustomerID |
|————|————–|———|————|
| 1 | John | 1 | 1 |
| 2 | Jane | NULL | NULL |
As you can see, all rows from the Customers table are included, but the columns from the Orders table are filled with NULL values for the non-matching rows.
Left Outer Join: When there are non-matching rows in the right table, the result will still include all rows from the left table, and the columns from the right table will be filled with NULL values.
Using the same example, if we perform a Left Outer Join on the Customers table and Orders table, we will get the following result:
| CustomerID | CustomerName | OrderID | CustomerID |
|————|————–|———|————|
| 1 | John | 1 | 1 |
| 2 | Jane | NULL | NULL |
As you can see, the result is the same as the Left Join, as there are no non-matching rows in the Customers table. However, if there were non-matching rows in the Orders table, the Left Outer Join would still include them in the result.
When to use Left Join and Left Outer Join
Now that we understand the difference between Left Join and Left Outer Join, the question is: when do we use each join?
– Use Left Join when you want to include all rows from the left table, and only matching rows from the right table. This join is useful when you want to retrieve data from related tables, even if there are no matching rows in the right table.
– Use Left Outer Join when you want to include all rows from the left table, and all matching rows from the right table. This join is useful when you want to retrieve data from related tables, and you want to include all the rows in the left table, even if there are no matching rows in the right table.
FAQs
Q: What happens when there are non-matching rows in the left table?
A: Both Left Join and Left Outer Join will return NULL values for the columns from the right table when there are non-matching rows in the left table.
Q: What is the syntax for Left Join and Left Outer Join?
A: The syntax for Left Join and Left Outer Join is as follows:
SELECT column(s)
FROM table1
LEFT JOIN table2 ON table1.column = table2.column;
SELECT column(s)
FROM table1
LEFT OUTER JOIN table2 ON table1.column = table2.column;
Q: What is the difference between Left Join and Inner Join?
A: The main difference between Left Join and Inner Join is how they handle non-matching rows. Left Join includes all rows from the left table, while Inner Join only includes matching rows from both tables.
Conclusion
In summary, Left Join and Left Outer Join are useful tools for combining data from two or more tables in a database. The key difference between these joins is how they handle non-matching rows. Left Join will include all rows from the left table and only matching rows from the right table, while Left Outer Join will include all rows from the left table and all matching rows from the right table, and NULL values for non-matching rows. Knowing when to use each join is critical to ensure that you retrieve the data you need from your database effectively.