On Wed, Mar 19, 2008 at 1:19 PM, Andrew Ballard <aballard@xxxxxxxxx> wrote:
> On Wed, Mar 19, 2008 at 1:04 PM, TG <tg-php@xxxxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > It seems that count(*) pulls all the data from the row then performs a count
> > increment whereas count(did) only pulls the 'did' column.
>
> Again, I don't believe COUNT(*) pulls any data. If there is a row, it
> simply counts it. The row could be full of NULLS (if allowed by your
> schema - yikes) and it will still be counted. I'd guess that COUNT(1)
> does the same thing. COUNT(did) does only examine the `did` column,
> but NULL values are excluded from the count.
You are correct, sir! COUNT(*) doesn't look into the data at all,
it just counts all rows. Keep in mind that COUNT(*) may very well
return a different result than the cardinality of the table, since
COUNT(*) couldn't care less if the row is unique or not.
> > I wonder if count(did) is the same speed as count(1) or if it will depend on
> > how much/what type of data is in 'did'.
> >
> >
> > I also wonder why count() takes a parameter. Isn't it always going to count
> > +1 for the row? I'll have to look that up sometime.
>
> It takes a parameter because it depends on what you want to count.
> COUNT(*) will return the number of rows matching the WHERE clause.
> COUNT(`column_name`) will return the number of non-NULL values in the
> column `column_name`. You could have a million rows in the table, but
> if every row has NULL in `column_name`, the COUNT() will return 0.
> There is also COUNT(DISTINCT `column_name`), which counts the number
> of distinct, non-NULL values in the column.
You can extend a SELECT COUNT(*) query almost exactly like you
would a basic SELECT query. Examples:
SELECT COUNT(*) FROM users WHERE username LIKE '%dan%';
SELECT COUNT(DISTINCT color) FROM products;
SELECT COUNT(*) FROM table LIMIT 0,1;
Any limits or the like on the query (such as in the last example)
will pretty much be ignored, though, because COUNT(*) only returns the
number of matching rows, not any other data whatsoever.
--
</Daniel P. Brown>
Forensic Services, Senior Unix Engineer
1+ (570-) 362-0283
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