MySQL在WHERE语句中使用COALESCE或IFNULL选择非NULL值

Question

I'm building a summary in a table that I self-join in order to count the rows corresponding on certain values (in TikiWiki CMS, tracker application). But I have a case where there are two columns that can have the same values, and I need SQL to take the first non-NULL value in a specific order.

Here is an example of the table tiki_tracker_item_fields where I work:

 itemId  | fieldId  | value   
==========================
41       | 236      | Paris   
41       | 213      | Paris  
14       | 236      |         
14       | 213      | Paris  
25       | 236      | Paris     
25       | 213      |       

In the query I want to count all lines that have "Paris" as value, either in field Id 236 or field ID 213. The result here should be: 3.

There is my request:

 SELECT COUNT(*) AS sp_actions           
FROM                                    
  `tiki_tracker_item_fields` AS actions 
WHERE                                   
  COALESCE(                             
    actions.fieldId = 236,              
    actions.fieldId = 213               
  )                                     
AND actions.value = 'Paris'                  

So this works when I have 'Paris' for field 236, but not when I have Paris for field 213.

And I'm not even sure I could use COALESCE this way, all examples I found put COALESCE in the SELECT statement.

Then there are the performance issues...

Answer 1

You can use count (distinct) with a case :

select 
    count(distinct case when value = 'Paris' then itemId end) cnt
from `tiki_tracker_item_fields`
where fieldId in (236,213);