On Wed, Mar 19, 2008 at 1:19 PM, Andrew Ballard [EMAIL PROTECTED] wrote:
On Wed, Mar 19, 2008 at 1:04 PM, TG [EMAIL PROTECTED] 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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