SQL: Alias Column Name for Use in CASE Statement

Question

Is it possible to alias a column name and then use that in a CASE statement? For example,

SELECT col1 as a, CASE WHEN a = \'test\' THEN \'yes\' END as value          


        

Answer 1

This:

SELECT col1 as a,
       CASE WHEN a = 'test' THEN 'yes' END as value 
  FROM table;

...will not work. This will:

SELECT CASE WHEN a = 'test' THEN 'yes' END as value
  FROM (SELECT col1 AS a
          FROM TABLE)

Why you wouldn't use:

SELECT t.col1 as a,
       CASE WHEN t.col1 = 'test' THEN 'yes' END as value 
  FROM TABLE t;

...I don't know.