SQL | Joining three or more tables

Joining three or more tables in SQL is a continuation of the same principles used when joining two tables. You essentially chain the joins, specifying the join conditions for each pair of tables.

Consider the following three tables:

users: user_id, username
orders: order_id, user_id, product_id
products: product_id, product_name

Example: To find out which user ordered which product, you need to join all three tables.

SELECT users.username, products.product_name
FROM users
JOIN orders ON users.user_id = orders.user_id
JOIN products ON orders.product_id = products.product_id;

Here's a breakdown:

First, users is joined with orders based on user_id.
The intermediate result is then joined with products based on product_id.

You can also combine different types of joins:

SELECT users.username, products.product_name
FROM users
LEFT JOIN orders ON users.user_id = orders.user_id
INNER JOIN products ON orders.product_id = products.product_id;

In this example:

We're using a LEFT JOIN between users and orders. This means that even if a user hasn't made any orders, they'll still appear in the intermediate result.
We then use an INNER JOIN with products. This means only those rows with a valid product will appear in the final result.

Notes:

Always ensure you're joining on indexed columns, especially in large tables, to maintain query performance.
Keep track of the type of each join as you chain them together, as the order and type can affect your results.
If dealing with tables from different databases or schemas, make sure to specify the full table name (e.g., database_name.table_name).
Always test your multi-table joins to ensure they're returning the expected results.

Joining Multiple Tables in SQL Query:

SELECT employees.employee_id, employees.employee_name, departments.department_name, projects.project_name
FROM employees
INNER JOIN departments ON employees.department_id = departments.department_id
INNER JOIN employee_projects ON employees.employee_id = employee_projects.employee_id
INNER JOIN projects ON employee_projects.project_id = projects.project_id;

How to Perform a Three-Table Join in SQL:

Use multiple INNER JOIN clauses to connect the necessary tables.

SELECT *
FROM table1
INNER JOIN table2 ON table1.column_id = table2.column_id
INNER JOIN table3 ON table2.column_id = table3.column_id;

Joining Four or More Tables in a Single SQL Query:

SELECT *
FROM table1
INNER JOIN table2 ON table1.column_id = table2.column_id
INNER JOIN table3 ON table2.column_id = table3.column_id
INNER JOIN table4 ON table3.column_id = table4.column_id;

SQL INNER JOIN with Three or More Tables:

Continue to use INNER JOIN for each additional table needed.

SELECT *
FROM table1
INNER JOIN table2 ON table1.column_id = table2.column_id
INNER JOIN table3 ON table2.column_id = table3.column_id
INNER JOIN table4 ON table3.column_id = table4.column_id;

Complex Multi-Table Joins in SQL Explained:

Complex joins involve multiple tables with intricate relationships.

SELECT *
FROM employees
INNER JOIN departments ON employees.department_id = departments.department_id
LEFT OUTER JOIN employee_projects ON employees.employee_id = employee_projects.employee_id
INNER JOIN projects ON employee_projects.project_id = projects.project_id
WHERE projects.status = 'Active';

Outer Join with Three or More Tables in SQL:

Use OUTER JOIN for tables where you want to retain unmatched rows.

SELECT *
FROM table1
LEFT OUTER JOIN table2 ON table1.column_id = table2.column_id
INNER JOIN table3 ON table2.column_id = table3.column_id;

Joining Tables Using Subqueries in SQL:

Subqueries can be used for more complex conditions.

SELECT *
FROM table1
INNER JOIN (
    SELECT column_id, column_name
    FROM table2
    WHERE condition
) AS subquery ON table1.column_id = subquery.column_id;