PHP and MySQL Web Development 4th Ed-tqw-_darksiderg

Retrieving Data from the Database
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which tells MySQL to put rows in the result table only if the customerid from the
customers table matches the customerid from the orders table.
By adding this join condition to the query, you actually convert the join to a different
type, called an equi-join.
Also notice the dot notation used to make it clear which table a particular column
comes from; that is, customers.customerid refers to the customerid column from the
customers table, and orders.customerid refers to the customerid column from the
orders table.
This dot notation is required if the name of a column is ambiguous—that is, if it
occurs in more than one table. As an extension, it can also be used to disambiguate column
names from different databases.This example uses a table.column notation, but
you can specify the database with a database.table.column notation, for example, to
test a condition such as
books.orders.customerid = other_db.orders.customerid
You can, however, use the dot notation for all column references in a query. Using this
notation can be a good idea, particularly when your queries begin to become complex.
MySQL doesn’t require it, but it does make your queries much more humanly readable
and maintainable. Notice that we followed this convention in the rest of the previous
query, for example, with the use of the condition
customers.name = ‘Julie Smith’
The column name occurs only in the table customers, so we do not really need to specify
what table it is from. MySQL will not be confused. For humans, though, the name on
its own is vague, so it does make the meaning of the query clearer when you specify it as
customer.name.
Joining More Than Two Tables
Joining more than two tables is no more difficult than a two-table join. As a general rule,
you need to join tables in pairs with join conditions.Think of it as following the relationships
between the data from table to table to table.
For example, if you want to know which customers have ordered books on Java (perhaps
so you can send them information about a new Java book), you need to trace these
relationships through quite a few tables.
You need to find customers who have placed at least one order that included an
order_item that is a book about Java.To get from the customers table to the orders
table, you can use the customerid as shown previously.To get from the orders table to
the order_items table, you can use the orderid.To get from the order_items table to
the specific book in the Books table, you can use the ISBN. After making all those links,
you can test for books with Java in the title and return the names of customers who
bought any of those books.