Combining using WHERE and JOIN

Using the combination of WHERE and JOIN, you can implement search and join operations in one SQL statement, so as to obtain the required data more conveniently. Here are the detailed steps:

1. Determine the table and fields to be queried: First, you need to determine the table to be queried and the fields to be returned. For example, suppose you want to query the customer table and the order table, and you need to return fields such as customer name, order number, and order date.

2. Use JOIN to connect the tables: Use the JOIN keyword to connect the customer table and the order table. The connection condition needs to be determined according to the relationship between the tables, usually using the common fields in the two tables to connect. For example, here you can join the Customers table with the Orders table using the customer ID field:

 

SELECT customers.customer_name, orders.order_id, orders.order_date
   FROM customers
   JOIN orders
   ON customers.customer_id = orders.customer_id

3. Search using WHERE: Use the WHERE clause to filter the required data. For example, here you need to search for customers whose customer name contains "John", you can use the LIKE operator and the wildcard % to achieve:

 SELECT customers.customer_name, orders.order_id, orders.order_date
   FROM customers
   JOIN orders
   ON customers.customer_id = orders.customer_id
   WHERE customers.customer_name LIKE '%John%'

4. Run query: Run a SQL query to get search results. The query results will contain all customers whose name contains "John" and their order information.

It should be noted that when using the combination of WHERE and JOIN to query, it is necessary to ensure that there is no conflict between the connection conditions and the search conditions. Because the search conditions may affect the results of the join conditions, resulting in wrong query results. Therefore, the query conditions need to be carefully checked to ensure that the correct results are obtained.

In short, using the combination of WHERE and JOIN can realize search and connection operations in one SQL statement, so as to obtain the required data more conveniently.