![]() ![]() Let’s look at a case in which a common field from the second table is used to left-join with the third but not with the first. Note that in this example, we used a common field from the first table to join both the second and the third tables. Finally, customers who have made purchases and received promotional messages (Customers 2 and 3) are also included in the result. We also have Customer 4 who has purchased a book but has not received any promotional messages, as well as Customer 5 who has received a promotional message but has not made any purchases. For example, Customer 1 is in the result of the join even though he has not made any purchases nor received the promotional message. To do this, we need to combine the data about customers, sales, and promotions.Īs you can see, by using a LEFT JOIN, we kept the records on all of our customers, regardless of purchase history or participation in the promotional campaigns. We want to analyze how our recent promotional campaign has impacted the behavior of our customers. Fortunately, the LEFT JOIN keyword can be used with multiple tables in SQL. ![]() Sometimes you need to LEFT JOIN more than two tables to get the data required for specific analyses. Now, let’s dive into more complicated cases! Multiple LEFT JOINs in One Query You may want to practice LEFT JOINs and other types of joins in our comprehensive SQL JOINs course or check out our SQL Practice track with multiple SQL exercises to practice various SQL constructions. idĪs you can see, the customers with no purchases in the given period have a NULL value for the sales date and the total amount spent, since their records are not present in the sales table. In the table below, the rows that would be present in INNER JOIN are highlighted in blue, while the yellow denotes additional records due to the LEFT JOIN. With this, we keep all the records about the customers, append the sales date, and calculate the total amount spent. S.date AS sales_date, sum(s.amount) AS total_spent SELECT c.id, c.first_name, c.last_name, c.gender, c.age, c.customer_since, To join these two tables and obtain the information we need for analysis, use the following SQL query: Here are the customers and sales tables from our bookstore. In this case, we want the result to include ALL customers–both active and inactive–by joining the data about our customers with the data about recent sales. Imagine that we run an online bookstore, and we want to compare the customers with orders in the last 6 months with those who were inactive during the same period. Alternatively, use a column from the second table that is fully populated with no NULL values, if there is one. However, first make sure that your second table doesn’t have records with NULL values in all columns except for the common field used for joining. In this case, you can add a WHERE clause to the query to select, from the result of the join, the rows with NULL values in all of the columns from the second table. First, it is very useful for identifying records in a given table that do not have any matching records in another. The LEFT JOIN is frequently used for analytical tasks. If you want to practice different types of joins, check out our interactive SQL JOINs course with over 90 hands-on exercises. If you want more information on SQL joins, check out this comprehensive guide. The general syntax for a LEFT JOIN is as follows: If a record from the right table is not in the left, it will not be included in the result. This implies that, if a specific row is present in the left table but not in the right, the result will include this row but with a NULL value in each column from the right. In contrast, a LEFT JOIN in SQL returns all records (or rows) from the left table and only the matched records (or rows) from the right. You might remember that an INNER JOIN returns only the records that are in both tables. Let’s start by reminding ourselves what a LEFT JOIN is and how to use it.
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