Getting confused by MySQL subqueries

I'm trying to learn how to do subqueries, and I'm really confused at what's wrong with this simple example.

开发者_开发技巧My first try was

SELECT COUNT(SELECT * FROM my_table);

but that didn't work (I guess because I need a temporary table?) so I tried this:

SELECT COUNT(items)
FROM (SELECT * FROM my_table) AS items;

Why do I get the following:

1054: Unknown column 'items' in 'field list'

You're getting the error because in this example items is a table (as it is an alias), not a column. Simplest solution is to use:

SELECT COUNT(*)
  FROM (SELECT * FROM my_table) AS items

Aggregate functions (IE: COUNT, MIN, MAX, AVG, etc) only work on column references, but some accept [table].* as a parameter.