MySQL：计算已知（或枚举）不同值的出现次数

Question

After looking at how to count the occurrences of distinct values in a field, I am wondering how to count the occurrences of each distinct value if the distinct values are known (or enumerated).

For example, if I have a simple table -

TrafficLight        Colour
------------        ------
1                   Red 
2                   Amber
3                   Red 
4                   Red 
5                   Green 
6                   Green 

where one column (in this case Colour) has known (or enumerated) distinct values, how could I return the count for each colour as a separate value, rather than as an array, as in the linked example.

To return an array with a count of each colour (using the same method as in the linked example), the query would be something like SELECT Colour COUNT(*) AS ColourCount FROM TrafficLights GROUP BY Colour , and return an array -

Colour          ColourCount 
------          -----------
Red             3
Amber           1
Green           2

What I would like to do is to return the count for each Colour AS a separate total (eg RedCount). How can I do this?

Answer 1

For mysql you can do so,by using expression in sum(expr) will result as boolean and you can count the occurrences for your colors individually

SELECT 
SUM(Colour = 'Red') AS RedCount, 
SUM(Colour = 'Amber') AS AmberCount, 
SUM(Colour = 'Green') AS GreenCount
FROM t 

Demo

Answer 2

Try this query:

SELECT
  (SELECT COUNT(*) AS ColourCount FROM tableA GROUP BY colour HAVING colour = 'Red') AS     red_lights,
  (SELECT COUNT(*) AS ColourCount FROM tableA GROUP BY colour HAVING colour = 'Green') AS green_lights,
  (SELECT COUNT(*) AS ColourCount FROM tableA GROUP BY colour HAVING colour = 'Amber') AS amber_lights
FROM
  tableA

Here is the Fiddle

Answer 3

You don't have to know the enumerated values:

SELECT Colour, count(*) as Frequency FROM t GROUP BY Colour ;

If your code gets used where 'amber' is reported as 'yellow', it still works.