How to count the number of grouped rows in mysql when I already count the total rows

问题

I have the below table and I want to do the following:

1. Count the number of times each item appears in the table

2. Count the DISTINCT number of items

3. Group the items by name

   +-------+---------+
   |  id   |  names  |
   +-------+---------+
   |   1   |  Apple  |
   |   2   |  Orange | 
   |   3   |  Grape  | 
   |   4   |  Apple  | 
   |   5   |  Apple  | 
   |   6   |  Orange | 
   |   7   |  Apple  | 
   |   8   |  Grape  |  
   +-------+---------+
   

   For the 1. and 3. points I have the following query which works quite well:

   SELECT * , 
      COUNT(names) as count_name, 
      FROM tbl_products WHERE type = '1' 
      GROUP BY names
   

   So I get:

   Apple  (4)
   Orange (2)
   Grape  (2)
   

Now I want to also count the number of grouped by rows and added a line to count the distinct elements, however there is some problem, since MySQL accepts the query but cannot output a result:

SELECT * , 
   COUNT(names) as count_name, 
   COUNT(DISTINCT names) as count_total 
   FROM tbl_products WHERE type = '1' 
   GROUP BY names

Can anyone advice what might be the problem?

EDIT: For more clearance I want to get a table like this:

    +-------+---------+------------+-------------+
    |  id   |  names  |  count_ctg | count_total |
    +-------+---------+------------+-------------+
    |   1   |  Apple  |      4     |      3      |
    |   2   |  Orange |      2     |      3      |
    |   3   |  Grape  |      2     |      3      |
    +-------+---------+------------+-------------+

回答1:

Why not just use the query you are using:

SELECT * , 
COUNT(names) as count_name, 
FROM tbl_products WHERE type = '1' 
GROUP BY names

This query achieves all three objectives.

1) You get a count of the number of each name value in count_name.

2) The number of distinct names values will be equal to the number of rows in the result set , since you are grouping by names. Pretty much any client-side MySQL DB connection library will enable you to retrieve this value.

3) You meet your third criteria of grouping by name by explictly using GROUP BY names

Of course the value for id in the result set is meaningless, you may want to only select names and count_names.

回答2:

You can get the count of distinct names in a subquery, then OUTER JOIN that thing back into your main query where you already solved for 1 and 3:

SELECT names , 
    COUNT(names) as count_name, 
    Total
FROM tbl_products 
    OUTER JOIN (SELECT count(DISTINCT names) as Total FROM tbl_products) t2 
WHERE type = '1' 
GROUP BY names

回答3:

1-.Count the number of times each item appears in the table:

SELECT names, count(names) FROM tbl_products WHERE type = '1' group by names

2-. How many distinct items exist in the table:

SELECT DISTINCT names FROM tbl_products WHERE type = '1'

3-. Group the items by name:

SELECT count(DISTINCT names) as Total FROM tbl_products WHERE type = '1'

As your last EDIT (ALL IN ONE):

SELECT id, names, count(names), total FROM tbl_products, (select count(distinct names) as total from tbl_products) as total WHERE type = '1' group by names

回答4:

You can use the SQL Windowing OVER()

This query returns the row_number() function as the id column in the results, and the over(...) for row_number requires an order by clause. You could order by whatever you want, but it most be ordered by something.

;WITH vwGroups (name, Quantity) AS 
(
    SELECT  name
    ,       COUNT(*)
    FROM    tbl_products
    GROUP BY name 
)
SELECT  ROW_NUMBER() OVER(ORDER BY Quantity DESC, name) AS id
,       name
,       Quantity AS count_name
,       COUNT(*) OVER () AS count_total
FROM    vwGroups

来源：https://stackoverflow.com/questions/28726329/how-to-count-the-number-of-grouped-rows-in-mysql-when-i-already-count-the-total