Left Join

A left join, also known as a left outer join, is a type of join operation in relational databases that returns all the rows from the left table and the matching rows from the right table, if any. If there is no match, the result will contain NULL values in the columns from the right table.

The syntax of a left join is as follows:

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SELECT column_name(s)
FROM table1
LEFT JOIN table2
ON table1.column_name = table2.column_name;

In the above syntax, table1 is the left table, table2 is the right table, and column_name refers to the columns that will be selected from the joined tables.

Using this syntax, we can write the following SQL statement to perform a left join between the pages table and the orders table:

SELECT p.*, o.*
FROM pages p
LEFT JOIN orders o
ON p.SessionID = o.SessionID;

In the context of the provided tables, suppose we want to retrieve a list of all the pages viewed by clients, including those who did not place an order. We can use a left join between the pages table and the orders table on the SessionID column to achieve this.

In the above statement, we are selecting all columns from the pages table and the Order table . The LEFT JOIN clause specifies that we want to retrieve all the rows from the pages table, even if there is no matching row in the orders table. The ON clause specifies the join condition, which is the SessionID column in both tables.

The result of the left join will include all the rows from the pages table, along with the matching rows from the orders table, if any. If there is no matching row in the orders table, the Orderno and Ordervalue columns will contain NULL values.