Getting the Number of Rows with a Group by Query

How do I COUNT rows of a GROUP BY query where a condition matches?

You can use conditional aggregation:

SELECT neighborhood, SUM(YEAR(NOW()) - p.birthyear) as under_18,
SUM(YEAR(NOW()) - p.birthyear BETWEEN 34 AND 42) as age_34_42
FROM persons p
GROUP BY neighborhood;

Getting the number of rows with a GROUP BY query

There is a nice solution in MySQL.

Add the keyword SQL_CALC_FOUND_ROWS right after the keyword SELECT :

SELECT SQL_CALC_FOUND_ROWS t3.id, a,bunch,of,other,stuff FROM t1, t2, t3 
WHERE (associate t1,t2, and t3 with each other)
GROUP BY t3.id
LIMIT 10,20

After that, run another query with the function FOUND_ROWS() :

SELECT FOUND_ROWS();

It should return the number of rows without the LIMIT clause.

Checkout this page for more information : http://dev.mysql.com/doc/refman/5.0/en/information-functions.html#function_found-rows

Count number of records returned by group by

You can do both in one query using the OVER clause on another COUNT

select
count(*) RecordsPerGroup,
COUNT(*) OVER () AS TotalRecords
from temptable
group by column_1, column_2, column_3, column_4

SELECT number of groups resulted from a GROUP BY query

SELECT count(*)
FROM (
SELECT 1 as dummy
FROM MyTable
WHERE Col2 = 'x'
GROUP BY Col1
) dt

No need to count rows in the sub-query, the result will be the same anyway.

How to get the total number of rows of a GROUP BY query?

The method I ended up using is very simple:

$query = 'SELECT a, b, c FROM tbl WHERE oele = 2 GROUP BY boele';
$nrows = $db->query("SELECT COUNT(1) FROM ($query) x")->fetchColumn();

Might not be the most efficient, but it seems to be foolproof, because it actually counts the original query's results.

How to use count and group by at the same select statement

This will do what you want (list of towns, with the number of users in each):

SELECT `town`, COUNT(`town`)
FROM `user`
GROUP BY `town`;

You can use most aggregate functions when using a GROUP BY statement
(COUNT, MAX, COUNT DISTINCT etc.)

Update:
You can declare a variable for the number of users and save the result there, and then SELECT the value of the variable:

DECLARE @numOfUsers INT
SET @numOfUsers = SELECT COUNT(*) FROM `user`;

SELECT DISTINCT `town`, @numOfUsers FROM `user`;

How to count how many rows inside a group by group meets a certain criteria

I would suggest using CASE WHEN (standard ISO SQL syntax) like in this example:

SELECT   a.category,
SUM(CASE WHEN a.is_interesting = 1 THEN 1 END) AS conditional_count,
COUNT(*) group_count
FROM a
GROUP BY a.category

This will sum up values of 1 and null values (when the condition is false), which comes down to actually counting the records that meet the condition.

This will however return null when no records meet the conditions. If you want to have 0 in that case, you can either wrap the SUM like this:

COALESCE(SUM(CASE WHEN a.is_interesting = 1 THEN 1 END), 0)

or, shorter, use COUNT instead of SUM:

COUNT(CASE WHEN a.is_interesting = 1 THEN 1 END)

For COUNT it does not matter what value you put in the THEN clause, as long as it is not null. It will count the instances where the expression is not null.

The addition of the ELSE 0 clause also generally returns 0 with SUM:

SUM(CASE WHEN a.is_interesting = 1 THEN 1 ELSE 0 END)

There is however one boundary case where that SUM will still return null. This is when there is no GROUP BY clause and no records meet the WHERE clause. For instance:

SELECT SUM(CASE WHEN 1 = 1 THEN 1 ELSE 0 END)
FROM a
WHERE 1 = 0

will return null, while the COUNT or COALESCE versions will still return 0.

Counting number of grouped rows in mysql

You need to do -

SELECT
COUNT(*)
FROM
(
SELECT
DISTINCT component
FROM
`multiple_sample_assay_abc`
WHERE
labref = 'NDQA201303001'
) AS DerivedTableAlias

You can also avoid subquery as suggested by @hims056 here



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