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LEFT, RIGHT and INNER JOINs
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The online store has ordered more products from a supplier because they were running low on stock. This means we have some free time until the shipment arrives.
Let's use this opportunity to learn something new! So far, you've been using the standard JOIN in SQL, but there are other types of joins you can use.
Here are the 4 main types of table joins:
INNER JOIN: Returns rows with matching values in both tables. This is the same as the standard JOIN you've been using:
LEFT JOIN: Returns all rows from the left table and the matching rows from the right table. If there are no matches, it returns NULL for the right table:
RIGHT JOIN: Returns all rows from the right table and the matching rows from the left table. If there are no matches, it returns NULL for the left table:
FULL JOIN: Returns all rows when there is a match in one of the tables. If there are no matches, it returns NULL for the missing values in the other table:
Before we dive into using these joins, let's check out the two tables we'll be working with. They contain details about courses and the students enrolled in them.
courses:
enrollments:
The syntax for using these types of joins is actually simple. Instead of the familiar JOIN or INNER JOIN, just specify LEFT JOIN or any other type of JOIN:
SELECT table1.column, table2.column
FROM table1 -- this is the LEFT table
LEFT JOIN table 2 -- this is the RIGHT table
ON table1.common_column = table2.common_column
Svep för att börja koda
Write a query to retrieve a list of all courses and the students enrolled, including courses with no registered students.
You need to fetch the following columns in this order:
courses.course_id,
courses.course_name,
courses.description,
enrollments.student_name,
enrollments.enrollment_date
Use the appropriate type of JOIN to solve this task!
Brief Instructions
- Retrieve the columns
courses.course_id,courses.course_name,courses.description,enrollments.student_name, andenrollments.enrollment_datefrom thecoursestable. - Use a LEFT JOIN to join the
enrollmentstable. - The common column for both tables is
courses.course_id = enrollments.course_id.
Lösning
Byt till skrivbordet för praktisk övningFortsätt där du är med ett av alternativen nedanTack för dina kommentarer!
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