Need help modifying a SQL View

Question

I have a SQL view created already & that view gives certain columns from a database table Data. Now, I want to modify that view in such a manner that it does not need to add any additional columns to the result & should match exactly with the same result as this view generated before. The only modification I want here is: There is a column called PayDate that the view takes from Data table. I need that column to have value depending on other column called OrderType. The Data table looks like :

Book    OrderType   PayDate      ValuDate
-------------------------------------------
FFF     CA          21/05/2015   NULL
BBB     CO          NULL         2/02/2014
EEE     IO          NULL         3/2/2014
QQQ     Dividend    02/02/2014   NULL

Currently, the view gives OrderType & Paydate as a result. Twist here is, for OrderType = 'CA' OR 'Dvidend' we have PayDate . For OrderType = 'CO' OR 'IO' we have ValuDate. Each time for 'CO' or 'IO' the value got is NULL. So here I want a CASE that will check if OrderType is 'CO' OR 'IO' it'll return ValuDate in PayDate column. And at places where OrderType is 'CA' or 'Dividend' give value of PayDate from Data table in PayDate column of the view.

PS : Do not want to add a ValuDate column here. All we need is just put the value of ValueDate in PayDate column of view if OrderType is 'CO' or 'IO'`

IF  EXISTS (SELECT * FROM sys.views WHERE object_id = OBJECT_ID(N'[dbo].[ABC]'))
BEGIN
    DROP VIEW [dbo].[ABC]
    PRINT '<<< DROPPED VIEW [dbo].[ABC] >>>'
END
GO  

SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO

CREATE VIEW [dbo].[ABC]
AS

    SELECT  ISNULL(Book, '') + '_' + ISNULL(OrderType,'') + '_'
            + ISNULL(CONVERT(VARCHAR(8), PayDate, 112), '') + '_'
            MAX(t.[InstrumentKey]) [InstrumentKey] ,
            MAX(t.[BookKey]) [BookKey] ,
            t.[OrderType],
            MAX(t.[InstrumentID]) [InstrumentID] ,
            t.PayDate ,
            FROM    dbo.Data t (NOLOCK)
    LEFT OUTER JOIN dbo.ExtInstrument i ON t.InstrumentKey = i.InstrumentKey

    WHERE [STATUS] = 'LIVE'

    GROUP BY Book ,
            OrderType ,
            PayDate ,
            BackToBackBook 


GO
PRINT '<<< CREATED VIEW [dbo].[ABC] >>>'

GRANT SELECT ON dbo.ABC TO INVENTORY_READERS
GO

PRINT '<<< GRANT SELECT ON  VIEW [dbo].[ABC] TO  INVENTORY_READERS  >>>'
GO

Answer 1

This is something which I have done. But I want to use 'UNION' & get the view modified. Can we use UNION in anyways to modify this view??

IF  EXISTS (SELECT * FROM sys.views WHERE object_id = OBJECT_ID(N'[dbo].[ABC]'))
BEGIN
    DROP VIEW [dbo].[ABC]
    PRINT '<<< DROPPED VIEW [dbo].[ABC] >>>'
END
GO  

SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO

CREATE VIEW [dbo].[ABC]
AS


SELECT  ISNULL(Book, '') + '_' + ISNULL(OrderType,'') + '_'
        + ISNULL(t.RIC, '') + '_'
        + CASE OrderType
                            WHEN OrderType = 'PNoteCorporateAction' OR 'PNoteDividendDraft' OR 'PNoteDividend' THEN ISNULL(CONVERT(VARCHAR(8), PayDate, 112), '') 
            ELSE ISNULL(CONVERT(VARCHAR(8), ValuDate, 112), '') 
          END AS PayDate + '_'  
        MAX(t.[InstrumentKey]) [InstrumentKey] ,
        MAX(t.[BookKey]) [BookKey] ,
        t.[OrderType],
        MAX(t.[InstrumentID]) [InstrumentID] ,
        t.PayDate ,
        FROM    dbo.Data t (NOLOCK)
LEFT OUTER JOIN dbo.ExtInstrument i ON t.InstrumentKey = i.InstrumentKey

WHERE [STATUS] = 'LIVE'

GROUP BY Book ,
        OrderType ,
        CASE WHEN OrderType = 'PNoteCorporateAction' OR 'PNoteDividendDraft' OR 'PNoteDividend' THEN PayDate ELSE ValueDate END AS PayDate,
        BackToBackBook 


GO
PRINT '<<< CREATED VIEW [dbo].[ABC] >>>'

GRANT SELECT ON dbo.ABC TO INVENTORY_READERS
GO

PRINT '<<< GRANT SELECT ON  VIEW [dbo].[ABC] TO  INVENTORY_READERS  >>>'
GO

Answer 2

Trying to follow what you are asking. Is this what you mean?

CREATE VIEW [dbo].[ABC]
AS

SELECT  ISNULL(Book, '') + '_' + ISNULL(OrderType,'') + '_'
        + ISNULL(CONVERT(VARCHAR(8), PayDate, 112), '') + '_'
        MAX(t.[InstrumentKey]) [InstrumentKey] ,
        MAX(t.[BookKey]) [BookKey] ,
        t.[OrderType],
        MAX(t.[InstrumentID]) [InstrumentID] ,
        CASE WHEN OrderType in ('CO','IO') THEN t.ValuDate ELSE t.PayDate END AS PayDate,
        FROM    dbo.Data t (NOLOCK)
LEFT OUTER JOIN dbo.ExtInstrument i ON t.InstrumentKey = i.InstrumentKey

WHERE [STATUS] = 'LIVE'

GROUP BY Book ,
        OrderType ,
        CASE WHEN OrderType in ('CO','IO') THEN t.ValuDate ELSE t.PayDate END ,
        BackToBackBook 

EDIT: I suspect the first part needs to be

SELECT  ISNULL(Book, '') + '_' + ISNULL(OrderType,'') + '_'
        + ISNULL(CONVERT(VARCHAR(8), CASE WHEN OrderType in ('CO','IO') THEN t.ValuDate ELSE t.PayDate END, 112), '') + '_'