SQL LEFT JOIN

What is LEFT JOIN in SQL?

A left join is a type of join in SQL that returns all the records from the left table and only those from the right table, which are matched to records in the left table.

For example, let us take the Customers table.

The Orders table for each customer could be viewed accordingly.

Now, let us join these two tables using the LEFT JOIN in SQL.

The joined table would look like this:

This means that a left join returns all the values from the left table (i.e., the ORDERS table in this case), plus matched values from the right table or NULL in case of no matching join predicate.

For example, look at the Venn diagram below.

Assume that you have two tables, Table A and Table B. A left join between Tables A and B would mean that all the records from Table A would be displayed, along with only those records from Table B, which are matched to Table A.

Syntax of LEFT JOIN

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LEFT JOIN in SQL Example

Student Table

Language_Marks Table

To perform a left join on these two tables, we will use the following SQL query:

Abstract of Code

The above query displays the student_name, age, and class columns from the student table, and the subject, marks columns from the language_marks table.

Thus, the left table is joined to the right table using the *r_no* field.

Why Do We Need LEFT JOIN in SQL?

A left join is useful when a user wants to extract all the records from the left table. Even if there are no matched records in the right table, all the records from the left table would be displayed.

In the Customers and Orders table examples, we see that a left join is established on the Orders table and returns all the specified rows from the left table.