SQL Left Join
A SQL LEFT JOIN is a type of join operation that combines rows from two tables based on a related column, returning all the rows from the left table and matching rows from the right table. If there is no match, NULL values are returned for the columns of the right table.
The syntax for a LEFT JOIN in SQL is as follows:
SELECT column_list FROM left_table LEFT JOIN right_table ON join_condition;
column_list represents the columns you want to select from the resulting joined table.
left_table is the table from which you want to retrieve all rows, and
right_table is the table you want to join with the left table.
join_condition specifies how the two tables are related. It typically uses a common column between the two tables to establish the relationship. For example, if you have a “users” table and an “orders” table, and you want to retrieve all users along with their corresponding orders (if any), you would perform a LEFT JOIN as follows:
SELECT users.user_id, users.name, orders.order_id, orders.order_date FROM users LEFT JOIN orders ON users.user_id = orders.user_id;
In this example,
users is the left table, and
orders is the right table. The join condition
users.user_id = orders.user_id ensures that the rows are matched based on the user ID column.
The result of the LEFT JOIN operation will include all rows from the
users table, whether or not there is a matching row in the
orders table. If a user has no corresponding orders, the columns related to the
orders table will have NULL values.
Note that the order of the tables in the join matters. In a LEFT JOIN, the left table is the one from which you want to retrieve all rows. If you want to retrieve all rows from the right table, you would use a RIGHT JOIN instead.