You will need dynamic SQL to achieve that.
What database are you using? What version?
In SQL Server, this would be one way to do it:
DECLARE @selectStmt nvarchar(4000),
SELECT @field =
WHEN fld_type1 = 'A' THEN 'VAL_ALPHA1'
WHEN fld_type1 = 'D' THEN 'VAL_DATE1'
SET @selectStmt = 'SELECT '+@field+' FROM table2 WHERE id = 8'
Beware that dynamic SQL pose some security issues.
Have a look at the following article to read more about Dynamic SQL:
The Curse and Blessings of Dynamic SQL