Aggregate queries.
COUNTING
SELECT COUNT(*) FROM table;
With a condition:
SELECT COUNT(*) FROM wp_posts WHERE post_type = 'post' AND post_status = 'publish';
That is how you audit content: how many published posts, how many products, how many orders.
SUMMING
SELECT SUM(column) FROM table WHERE condition;
Useful for order totals over a period.
GROUPING
SELECT post_type, COUNT(*) FROM wp_posts GROUP BY post_type;
That returns a count per type, which immediately shows how much of the table is revisions.
It is one of the most useful diagnostic queries available.
ORDERING THE RESULT
Add ORDER BY the count descending to see the largest groups first.
WHY THIS MATTERS
Before cleaning up, count what you are about to remove. A count is safe and tells you the scale.
Running COUNT with the same WHERE clause as a planned DELETE is the discipline that prevents accidents.
OTHER AGGREGATES
AVG, MIN and MAX, all following the same pattern.
