来自另一个案例表达式的案例表达式

Question

I have sample of SQL code, that returns number of weeks

,CASE       WHEN dateadd(MONTH, datediff (MONTH,AgentStart, getdate()), AgentStart) > getdate()
            THEN datediff(MONTH, AgentStart, getdate()) - 1
            ELSE datediff(MONTH, AgentStart, getdate())
            END as 'Month'

I keep my head off how I can now add a new column that will give me this result (from same table):

,CASE       WHEN **MONTH FROM CASE STATEMENT ABOVE** BETWEEN 0 AND 3 THEN 'Youngest' 
            WHEN **MONTH FROM CASE STATEMENT ABOVE** BETWEEN 4 AND  5 THEN 'Young' 
            WHEN **MONTH FROM CASE STATEMENT ABOVE** BETWEEN 6 AND 11 THEN 'Medium' 
            ELSE 'Old'
            END AS "Status"

Answer 1

You could use a subquery:

SELECT `month`,
        CASE WHEN `month` BETWEEN 0 AND 3 THEN 'Youngest' 
             WHEN `month` BETWEEN 4 AND  5 THEN 'Young' 
             WHEN `month` BETWEEN 6 AND 11 THEN 'Medium' 
             ELSE 'Old'
        END AS Status
FROM   (SELECT CASE  
               WHEN dateadd(MONTH, datediff (MONTH,AgentStart, getdate()), AgentStart) > getdate()
               THEN datediff(MONTH, AgentStart, getdate()) - 1
               ELSE datediff(MONTH, AgentStart, getdate())
               END as `Month`
       FROM    mytable) t

Answer 2

You may write an inline view to get this done

Eg:

select case WHEN x.month_value BETWEEN 0 AND 3 THEN 'Youngest' 
            WHEN x.month_value  BETWEEN 4 AND  5 THEN 'Young' 
            WHEN x.month_value  BETWEEN 6 AND 11 THEN 'Medium' 
            ELSE 'Old'
            END AS "Status"
       ,--rest of the columns from the inner query...
  from (
    select CASE 
               WHEN dateadd(MONTH, datediff (MONTH,AgentStart, getdate()), AgentStart) > getdate()
                THEN datediff(MONTH, AgentStart, getdate()) - 1
                ELSE datediff(MONTH, AgentStart, getdate())
                END as month_value --naming it as month_value
    from <table>
       )x