SQL Joins
A JOIN clause is used to combine rows from two or more tables, based on a related column between them.
Let’s look at a selection from the “Orders” table:
Then, look at a selection from the “Customers” table:
Notice that the “CustomerID” column in the “Orders” table refers to the “CustomerID” in the “Customers” table. The relationship between the two tables above is the “CustomerID” column.
Then, we can create the following SQL statement (that contains an INNER JOIN), that selects records that have matching values in both tables:
SELECT Orders.OrderID, Customers.CustomerName, Orders.OrderDate
FROM Orders
INNER JOIN Customers ON Orders.CustomerID=Customers.CustomerID;
and it will produce something like this:
Different Types of SQL JOINS
Here are the different types of the JOINs in SQL:
– INNER JOIN: Returns records that have matching values in both tables
– LEFT (OUTER) JOIN: Returns all records from the left table, and the matched records from the right table
– RIGHT (OUTER) JOIN: Returns all records from the right table, and the matched records from the left table
– FULL (OUTER) JOIN: Returns all records when there is a match in either left or right table
Course Video
SQL JOINS are used to combine rows from two or more tables based on related columns. They allow you to query and retrieve data that spans multiple tables.
- INNER JOIN: Returns records with matching values in both tables.
- LEFT JOIN (LEFT OUTER JOIN): Returns all records from the left table and matching records from the right table.
- RIGHT JOIN (RIGHT OUTER JOIN): Returns all records from the right table and matching records from the left table.
- FULL JOIN (FULL OUTER JOIN): Returns all records when there is a match in either table.
The basic syntax for an INNER JOIN is:
sqlCopy codeSELECT table1.column1, table2.column2 FROM table1 INNER JOIN table2 ON table1.common_column = table2.common_column;
Example:
sqlCopy codeSELECT Employees.name, Departments.name FROM Employees INNER JOIN Departments ON Employees.dept_id = Departments.id;
Yes, you can use aggregate functions like SUM(), COUNT(), and AVG() with JOINS. Example:
sqlCopy codeSELECT Departments.name, COUNT(Employees.id) AS EmployeeCount FROM Employees INNER JOIN Departments ON Employees.dept_id = Departments.id GROUP BY Departments.name;
- Forgetting the ON clause, which defines the relationship between tables.
- Using ambiguous column names without specifying the table (e.g., id instead of table1.id).
- Joining on non-indexed columns, which can slow query performance.
Combine multiple JOINs to work with more than two tables. Example:
sqlCopy codeSELECT Orders.id, Customers.name, Products.name FROM Orders INNER JOIN Customers ON Orders.customer_id = Customers.id INNER JOIN Products ON Orders.product_id = Products.id;
Yes, explore our detailed SQL JOINS Tutorials Online for examples and step-by-st
- Use indexed columns for JOIN conditions.
- Avoid unnecessary columns in the SELECT statement.
- Use EXPLAIN to analyze and optimize the query performance.
- INNER JOIN: Returns only rows where there is a match in both tables.
- OUTER JOIN (LEFT, RIGHT, FULL): Includes unmatched rows from one or both tables.
Yes, JOINS can be combined with subqueries to create powerful queries. Example:
sqlCopy codeSELECT Employees.name, (SELECT COUNT(*) FROM Orders WHERE Orders.employee_id = Employees.id) AS OrderCount FROM Employees;
