Inner Joins

A join that identifies combinations of matching rows from two tables is called an inner join. The joining is
done by connecting one table, using the primary key, another table that references it with a foreign key.
Inner joins may be written using two different syntaxes. One syntax lists the tables to be joined separated
by a comma. The other uses the INNER JOIN keywords.
 Comma Separated Inner Join
To combine data from two or more tables to find the detail of records referenced in a foreign key , a table
join must be performed. The most common way to do this is by specifying a list (comma separated) of
tables in the FROM clause of the SELECT and using the WHERE clause to indicate the relationship between
the tables.
A simple question that might be asked about the information in the world database is, “What are the
country and corresponding capital cities (along with ID)?” To answer that question, the following two
individual queries could be executed…

mysql> SELECT Name, Capital
-> FROM Country;
+---------------+---------+
| Name | Capital |
+---------------+---------+
| Afghanistan | 1 |
...
| Sweden | 3048 |
...
| Germany | 3068 |
...
| United States | 3813 |
...
+---------------+---------+
239 rows in set (#.## sec)


mysql> SELECT Id, Name FROM
-> City;
+------+------------+
| Id | Name |
+------+------------+
| 1 | Kabul |
...
| 3048 | Stockholm |
...
| 3068 | Berlin |
...
| 3813 | Washington |
...
+------+------------+
4079 rows in set (#.## sec)


…or they can be joined into a multi-table query. This is accomplished by using a join condition. A join
condition is supplied in the WHERE clause (for comma joins) or by specifying the ON or USING keyword.
Notice that the column Capital (from the Country table) contains the same data as the Id column (from
City). Therefore, these two queries can be joined where the Capital and Id are the same;


mysql> SELECT Country.Name, City.Name, City.Id FROM Country, City
       -> WHERE Country.Capital = City.Id;
+----------------------+------------------+------+
| Name | Name | Id |
+----------------------+------------------+------+
| Afghanistan | Kabul | 1 |
...
| Sweden | Stockholm | 3048 |
...
| Germany | Berlin | 3068 |
...
| United States | Washington | 3813 |
...
+----------------------+------------------+------+
232 rows in set (#.## sec)