PostgreSQL – LEFT JOIN

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A LEFT JOIN is used to retrieve data from two or more tables based on a specified condition, while also including all the rows from the left (or first) table, even if there is no match in the right (or second) table. This means that even if there is no matching row in the right table, the row from the left table will still be included in the result set.

The basic syntax of a LEFT JOIN in PostgreSQL is as follows:

SELECT column_list FROM left_table LEFT JOIN right_table ON join_condition;

Here’s a breakdown of the components:

  • SELECT column_list: This is the list of columns you want to retrieve from the tables.
  • left_table: The table from which you want to retrieve data and which will be on the left side of the join operation.
  • right_table: The table you want to join with the left table, which will be on the right side of the join operation.
  • ON join_condition: This is the condition that specifies how the tables should be joined. It defines the relationship between the columns in the left and right tables.

Example: Consider two tables: “orders” and “customers.” You want to retrieve a list of all orders and the corresponding customer information, including orders without matching customers. The following query achieves this using a LEFT JOIN:

SELECT o.order_id, o.order_date, c.customer_name FROM orders o LEFT JOIN customers c ON o.customer_id = c.customer_id;

Above query retrieves the order ID and date from the “orders” table and the customer name from the “customers” table. The LEFT JOIN ensures that all rows from the “orders” table are included in the result, even if there is no matching customer in the “customers” table.

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