MySQL group_concat() ordering by case statement values

问题

In a MySQL group_concat() clause, I'm trying to order the resulting values of a case statement. The following query configuration properly orders things.name but does not order the 'Non-US' or 'Unknown' values within the same context.

SELECT 
  things.id
  ,group_concat(DISTINCT 
    CASE
    WHEN things.name <> 'United States' THEN 'Non-US'
    WHEN things.name IS NULL THEN 'Unknown'
    ELSE things.name
    END
  ORDER BY name SEPARATOR ', ')
FROM things
GROUP BY things.id

I want to do something like this, but it's not working:

SELECT 
  things.id
  ,group_concat(DISTINCT 
    (CASE
    WHEN things.name <> 'United States' THEN 'Non-US'
    WHEN things.name IS NULL THEN 'Unknown'
    ELSE things.name
    END) AS new_name
  ORDER BY new_name SEPARATOR ', ')
FROM things
GROUP BY things.id

Is there a way to sort by "new_name" without using sub-queries/ nested queries?

回答1:

You can accomplish this by ordering by column position instead of column name.

For your case ORDER BY 1 should work.

SELECT 
  things.id
  ,group_concat(DISTINCT 
    CASE
    WHEN things.name <> 'United States' THEN 'Non-US'
    WHEN things.name IS NULL THEN 'Unknown'
    ELSE things.name
    END
  ORDER BY 1 SEPARATOR ', ')
FROM things
GROUP BY things.id

回答2:

A session variable might work, but I am not sure the order of evaluation within a GROUP_CONCAT():

SELECT 
  things.id
  , group_concat(DISTINCT 
      @new_name := (CASE
        WHEN things.name <> 'United States' THEN 'Non-US'
        WHEN things.name IS NULL THEN 'Unknown'
        ELSE things.name
        END
      ) ORDER BY @new_name SEPARATOR ', ')
FROM things
GROUP BY things.id
;

If that doesn't work, you could try doing the assignment in the ORDER BY section instead, and just using @new_name in the pre-ORDER BY.

来源：https://stackoverflow.com/questions/31303035/mysql-group-concat-ordering-by-case-statement-values

标签

mysql

sql

sql-order-by

group-concat