How to check the sql three-table association query

SQL three-table association query involves the following steps: Select the table join type (INNER JOIN, LEFT JOIN, RIGHT JOIN, FULL JOIN) Use the ON keyword to specify the association conditions (left table field = right table field) to execute the query using the following syntax: SELECT FROM

ON ON

How to execute SQL three-table association query?

An association query is used to merge data from multiple tables. Using SQL to perform a three-table association query requires the following steps:

1. Table join type:

Select the connection type corresponding to your associated requirements:

• INNER JOIN: Returns only the public rows of two tables.
• LEFT JOIN: Returns all rows in the left table and rows matching the right table. If there is no match, return NULL.
• RIGHT JOIN: Returns all rows in the right table and rows matching the left table. If there is no match, return NULL.
• FULL JOIN: Returns all rows in both tables, even if there is no match.

2. Related conditions:

Specify the association conditions between join tables, usually using the ON keyword:

 <code class="sql">ON  = </code>

3. Query syntax:

Execute associated queries using the following syntax:

 <code class="sql">SELECT  FROM    ON    ON </code>

Example:

Suppose we have the following three tables:

• Customers: id , name , city
• Orders: id , customer_id , product_id , quantity
• Products: id , name , price

To find all orders and product information for a customer, you can use the following query:

 <code class="sql">SELECT Customers.name AS CustomerName, Products.name AS ProductName, Orders.quantity FROM Customers INNER JOIN Orders ON Customers.id = Orders.customer_id INNER JOIN Products ON Orders.product_id = Products.id WHERE Customers.name = 'John Doe';</code>

Notes:

• Make sure the data type of the connection field is the same.
• If multiple association conditions exist, they can be grouped using brackets.
• You can also specify the connection field (applicable to multiple public fields) using the USING keyword.

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