Left Join Vs Left Outer Join Sql

When it comes to database management, a common challenge is joining tables together to extract the desired data for analysis. JOINs in SQL are used to combine data from two or more tables based on a related column between them. Although JOINs come in different types, left join and left outer join are among the most popular ones. In this article, we will discuss the differences between left join and left outer join in SQL and when to use them.

Left Join in SQL

A left join is a type of join in SQL that returns all the rows from the left table and the matching rows from the right table. If there are no matches in the right table, NULL values are returned. The syntax for the left join is as follows:

SELECT column_list FROM table1 LEFT JOIN table2 ON table1.column = table2.column;

In the above syntax, column_list represents the name of the columns to be selected, table1 and table2 are the names of the tables to be joined, and column is the column common to both tables.

Let’s take an example to understand the left join in SQL better. Suppose we have two tables, Table1 and Table2, as shown below:

Table1

| ProductID | ProductName | UnitPrice |
|———–|————-|———–|
| 1 | Apple | 0.5 |
| 2 | Banana | 0.3 |
| 3 | Orange | 0.6 |
| 4 | Pineapple | 1.5 |

Table2

| ProductID | SupplierName |
|———–|—————-|
| 1 | Supplier1 |
| 2 | Supplier1 |
| 4 | Supplier2 |

If we want to retrieve all the products and their respective supplier names (if they have any), we can use a left join as follows:

SELECT Table1.ProductName, Table2.SupplierName
FROM Table1
LEFT JOIN Table2
ON Table1.ProductID = Table2.ProductID;

The output of this query will be:

As we can see from the output, the left join returns all the rows from the left table (Table1) and the matching rows from the right table (Table2), and NULL is returned for those rows where there is no match in the right table.

Left Outer Join in SQL

A left outer join is a type of join in SQL that returns all the rows from the left table and the matching rows from the right table. In addition, it also returns the rows from the left table that do not have any matches in the right table. The syntax for the left outer join is as follows:

SELECT column_list FROM table1 LEFT OUTER JOIN table2 ON table1.column = table2.column;

In the above syntax, column_list represents the name of the columns to be selected, table1 and table2 are the names of the tables to be joined, and column is the column common to both tables.

Let’s continue with the previous example to understand the left outer join in SQL better. If we use a left outer join to retrieve all the products and their respective supplier names (if they have any), we can use the following query:

SELECT Table1.ProductName, Table2.SupplierName
FROM Table1
LEFT OUTER JOIN Table2
ON Table1.ProductID = Table2.ProductID;

The output of this query will be:

As we can see from the output, the left outer join returns all the rows from the left table (Table1) and the matching rows from the right table (Table2). In addition, it also returns the rows from the left table that do not have any matches in the right table, such as the Orange product in this example.

Differences Between Left Join and Left Outer Join in SQL

Now that we have discussed the left join and left outer join in SQL, let’s compare them based on their differences:

1. Result sets: The main difference between the left join and left outer join is the result set. The left join returns only the matching rows from the right table, while the left outer join returns all the rows from the left table along with the matching rows from the right table and the non-matching rows from the left table.

2. NULL values: Another difference between the left join and left outer join is the NULL values. In the left join, NULL values are returned for the rows that do not have any matches in the right table. In the left outer join, NULL values are returned for the rows that do not have any matches in the right table, as well as the rows that do not have any matches in the left table.

3. Syntax: The syntax for the left join and left outer join is slightly different, with the left outer join having the keyword ‘OUTER’ between LEFT and JOIN.

When to Use Left Join and Left Outer Join in SQL

Now that we understand the differences between left join and left outer join, let’s discuss when to use each of these joins.

Left join is used when you only need the matching rows from the right table.

For example, if we want to retrieve a list of products and their order details (if they have any), we can use a left join as follows:

SELECT Table1.ProductName, SUM(Table2.Quantity) as TotalQuantity
FROM Table1
LEFT JOIN Table2
ON Table1.ProductID = Table2.ProductID
GROUP BY Table1.ProductName;

In this example, we only need the orders that match the products in Table1, so we use a left join to retrieve only the matching rows.

Left outer join is used when you need all the rows from the left table, including those that do not have any matches in the right table.

For example, if we want to retrieve a list of products and their respective supplier names (if they have any), we can use a left outer join as follows:

SELECT Table1.ProductName, Table2.SupplierName
FROM Table1
LEFT OUTER JOIN Table2
ON Table1.ProductID = Table2.ProductID;

In this example, we need all the products, along with their corresponding supplier names, even if some products do not have any suppliers yet. Therefore, we use a left outer join to retrieve all the rows from Table1.

FAQs:

Q1. What is a join in SQL?

A join in SQL is used to combine data from two or more tables based on a related column between them.

Q2. What are the types of join in SQL?

The types of join in SQL are INNER JOIN, LEFT JOIN, RIGHT JOIN, FULL OUTER JOIN, and CROSS JOIN.

Q3. What is the difference between LEFT JOIN and LEFT OUTER JOIN?

The main difference between LEFT JOIN and LEFT OUTER JOIN is that LEFT JOIN returns only the matching rows from the right table, while LEFT OUTER JOIN returns all the rows from the left table along with the matching rows from the right table and the non-matching rows from the left table.

Q4. When should I use LEFT JOIN?

LEFT JOIN is used when you only need the matching rows from the right table.

Q5. When should I use LEFT OUTER JOIN?

LEFT OUTER JOIN is used when you need all the rows from the left table, including those that do not have any matches in the right table.

Conclusion:

In conclusion, left join and left outer join are two of the most common types of SQL joins used for combining data from multiple tables. While left join returns only the matching rows from the right table, left outer join also includes the non-matching rows from the left table. Understanding the differences between these joins and when to use each of them will help in retrieving the desired data for analysis.