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

It should work. Try this

Select * from
              (select col1, col2, case when 1=1 then 'ok' end as alias_col
               from table)
        as tmp_table
order by 
       case when @sortBy  = 1 then tmp_table.alias_col end asc

Answer 2

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.

Answer 3

Not in MySQL. I tried it and I get the following error:

ERROR 1054 (42S22): Unknown column 'a' in 'field list'

Answer 4

In MySql, alice name may not work, therefore put the original column name in the CASE statement

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

Sometimes above query also may return error, I don`t know why (I faced this problem in my two different development machine). Therefore put the CASE statement into the "(...)" as below:

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

Answer 5

Nor in MsSql

SELECT col1 AS o, e = CASE WHEN o < GETDATE() THEN o ELSE GETDATE() END 
FROM Table1

Returns:

Msg 207, Level 16, State 3, Line 1
Invalid column name 'o'.
Msg 207, Level 16, State 3, Line 1
Invalid column name 'o'.

However if I change to CASE WHEN col1... THEN col1 it works