Database Essentials - Joined At The Hip
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Most databases also allow you to query two or more tables at a time, and display a combined set of results. This is technically referred to as a "join", since it involves "joining" different tables at specific points to create new views of the data.
Here's an example of a simple join:
sql> SELECT names.CustomerID, FirstName, LastName, StreetAddress, City,
State, Zip, Country FROM names, addresses WHERE names.CustomerID =
addresses.CustomerID;
+------------+-----------+----------+------------------------------+---------------+-------+--------+---------+
| CustomerID | FirstName | LastName | StreetAddress | City | State | Zip | Country |
+------------+-----------+----------+------------------------------+---------------+-------+--------+---------+
| 234673 | John | Doe | 12, Some Street | Nowheresville | ZZ | 748202 | 56 |
| 734736 | Julius | Caesar | The Palace Behind The Lake |Ancient Rome | NA | 0 | 0 |
| 1243 | Daffy | Duck | Next Door To The Rabbit Hole | Toontown | TT | 98765 | 0 |
+------------+-----------+----------+------------------------------+---------------+-------+--------+---------+
In this case, the "names" and "addresses" tables have been joined
together through the common column "CustomerID".
And you can also nest one query within another, such that the result of the inner query provides data for the outer query. Such a query is referred to as a "subquery", and it allows a great deal of flexibility when formulating long and complex queries.
Let's suppose you wanted to find out the names and addresses of all customers with an account balance greater than $1000.00
sql> SELECT FirstName, LastName, StreetAddress, City, State, Zip, Country
FROM names, addresses WHERE names.CustomerID = addresses.CustomerID AND
names.CustomerID IN (SELECT CustomerID FROM balance WHERE AccountBalance >=
1000);
In this case, SQL will first execute the inner query
sql> SELECT CustomerID FROM balance WHERE AccountBalance >= 1000;+------------+| CustomerID |+------------+| 734736 || 234673 |+------------+
and then assign the return value(s) to the outer query, which will display the result.
sql>SELECT FirstName, LastName, StreetAddress, City, State, Zip, Country
FROM names, addresses WHERE names.CustomerID = addresses.CustomerID AND
names.CustomerID IN ('734736', '234673');
+-----------+----------+----------------------------+---------------+-------+--------+---------+
| FirstName | LastName | StreetAddress | City | State | Zip | Country |
+-----------+----------+----------------------------+---------------+-------+--------+---------+
| John | Doe | 12, Some Street | Nowheresville | ZZ | 748202 | 56 |
| Julius | Caesar | The Palace Behind The Lake | Ancient Rome | NA | 0 | 0 |
+-----------+----------+----------------------------+---------------+-------+--------+---------+
There is a limit on the number of sub-queries you can use in a single
SQL statement, but it's usually quite a comfortable number.
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