MySQL中SELECT语句中的CASE条件

Question

I have CASE condition in SELECT. Below is the query condition

The query is from a view

SELECT expt_id,stain_type,control_stain, test_stain
   CASE WHEN stain_type = 'Blue' THEN control_stain = 'NA'
   CASE WHEN stain_type = 'Hemat' THEN test_stain = 'NA'
FROM experiment_results__view

Here the value 'NA' has to be populated for the condition of stain_type. Would appreciate your help with suggestions on how to write this query

Thanks!!

Answer 1

I expect you want to use something in the line:

SELECT 
   expt_id,
   stain_type,
   CASE WHEN stain_type = 'Blue' THEN 'NA' ELSE NULL END control_stain, 
   CASE WHEN stain_type = 'Hemat' THEN 'NA' ELSE NULL END test_stain
FROM experiment_results__view

to get your value of 'NA' into columns named control_stain and test_stain. But if you want to override the existing value in those columns with the value 'NA' in just those cases, then use:

SELECT 
   expt_id,
   stain_type,
   CASE WHEN stain_type = 'Blue' THEN 'NA' ELSE control_stain END control_stain, 
   CASE WHEN stain_type = 'Hemat' THEN 'NA' ELSE test_stain END test_stain
FROM experiment_results__view

If you're intending an update of your table then you should rewrite this to:

UPDATE 
   experiment_results__view 
SET 
   control_stain = CASE WHEN stain_type = 'Blue' THEN 'NA' ELSE control_stain END, 
   test_stain = CASE WHEN stain_type = 'Hemat' THEN 'NA' ELSE test_stain END
-- if you don't want to update the complete table add an WHERE clause
WHERE
   <some condition>

Answer 2

you need to END each case statement

SELECT expt_id,stain_type,
   CASE WHEN stain_type = 'Blue' THEN 'NA' ELSE control_stain END AS 'Control Stain'
   CASE WHEN stain_type = 'Hemat' THEN 'NA' ELSE test_stain END AS 'Test Stain'
FROM experiment_results__view

if you are intending to update the table you can do an update query

UPDATE 
    experiment_results__view
SET 
    control_stain = CASE WHEN stain_type = 'Blue'  THEN 'NA' ELSE control_stain END,
    test_stain    = CASE WHEN stain_type = 'Hemat' THEN 'NA' ELSE test_stain END