Assume there are 261 nations, 7 religions, and 393 entries in the Religions table. The world population is ~5.99 billion. There are 190 Christain nations. Suppose I want to know the number of people of various faiths. What would these querries produce?

    select sum(pop), religion from Religions, nations;

    select sum(pop), religion from Religions, nations where nation.code = Religion.code;

    select sum(pop), religion from Religions, nations where nation.code = Religion.code group by religion;

    select sum(pop * Percent) as Count, Religion from Religions, nations, where nation.code = Religion.code group by religion order by Count desc;

    select count(religion) as Count, religion from Religions order by Count desc;

    select count(religion) as Count, religion from Religions group by religion order by Count desc;

Which querry produces this?

+-------+------------+
| Count | religion   |
+-------+------------+
|   190 | Christian  |
|    95 | Muslim     |
|    33 | indigenous |
|    23 | Orthodox   |
|    22 | Buddhism   |
|    20 | Hindu      |
|    10 | Jewish     |
+-------+------------+
7 rows in set (0.01 sec)