8/13/2023 0 Comments Sqlite inner join explained![]() ON employees.id = jan_bonuses.employee_id Īs expected, the result includes all employees. Our result should include all employees, no matter if they received a bonus or not. We want to join these tables so we can see who received bonuses in January. We have a table with employees info and want to match it with the table that lists all the bonuses paid in January ( jan_bonuses). SQL LEFT JOINs In Use Example with employees And now let’s look into some more examples of working with LEFT JOINs. Simple, isn’t it? You can practice LEFT JOINs in this interactive SQL JOINs course. If there is no match, it still keeps the row from the left table and puts NULL in the corresponding columns of the right table (customer #2 in our example). If there are several matches (like in our case with customer #3), it duplicates the row in the left table to include all records from the right table. If there is a match, it adds data from the right table to the corresponding row of the left table. Take a look at the animation: How does LEFT JOIN work?įirst of all, the database looks into each row of the left table and searches for a match in the right table based on the related columns. In this query, we also use ORDER BY to order the output by customer’s ID, but this is optional.Īfter running this query, you’ll get the following table. (Here, it’s id from customers and customer_id from orders). ON – Use this keyword to indicate the columns that will be used to join the tables, i.e.LEFT JOIN – Write the name of the second (right) table (i.e.FROM – Put the name of the left table, the one where you want to keep all the records (i.e.SELECT – Start by listing the columns (from both tables) that you want to see in the result set (here we select all columns using *).Let’s go through the syntax of LEFT JOIN: To join these two tables while keeping all the records of the customers table, you can use the following SQL query: Here are our customers and orders tables. Let’s see how it works with the customers and orders example mentioned above. If there is no match for a specific record, you’ll get NULLs in the corresponding columns of the right table. LEFT JOIN, also called LEFT OUTER JOIN, returns all records from the left (first) table and the matched records from the right (second) table. If you are not sure which JOIN type you need in a particular case, check out our video tutorial on SQL LEFT, RIGHT, and FULL JOINs. This is the JOIN type that we’ll focus on in this article. In this case, you’ll use a LEFT JOIN, which combines data from two tables so that all rows from the left (first) table are included in the result. ![]() We may want to see information about all customers in our result set, even if they had no orders in a specific time period. There are cases when you want to keep rows from the first table that don’t have the corresponding records in the second table. ![]() This type of SQL JOIN keeps only those rows that are present in both tables in our case, you’ll see a row in the result set only if the customer’s ID is in the customers table as well as the orders table. If you’re interested in customers that have only placed orders during a specific time period, you’ll use a simple JOIN (also called INNER JOIN). You’ll need to choose the right one based on the rows you want to keep in the result set. However, it’s important to know that there are different types of JOINs in SQL. This will give you order and customer details for each customer. ![]() For example, if you have a table with customer information (customer ID, first name, last name, etc.) and another table with the order data for a specific period (order ID, date, customer ID, product ID, etc.) you can join these two tables based on the columns that store customer ID numbers. In SQL, you use JOIN to combine data from two tables based on a column with matching values. It has over 80 practical exercises, and covers all the different types of JOINs, including LEFT JOIN. The best way to review SQL JOINs is our interactive SQL JOINs course. ![]()
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