Inserting Values Intoto Only Column Based On A Condition

Hi,
I have at table as foolows
Table Cat3
{

ID,
Update datetime
}


and also have a master table as follows

Table Master
{

ID,
Cat1 Datetime,
Cat2 datettime
}


My requirement is to alter themaster table schema i.e to add a column with the name as of the table name i.e Cat3 and will lok lie as foolows
Table Master
{

ID,
Cat1 Datetime,
Cat2 datettime,
Cat3 Datetime
}

I would like to insert the data to this column. The sample output is as follows

Before insertng the data of table ;Cat3' into the Master table
Table Master
{

Id Cat1 Cat2
---------------------------------
1 D1 D2
2 D2 NULL
3 D3 D4
}


And The Cat3 table data is as follows

Table Cat3
{
ID Update
--------------------------
1 D5
3 D6
4 D7

}

The final putput of the master table should be as follows

Table Master
{

Id Cat1 Cat2 Cat3
------------------------------------------------------
1 D1 D2 D5
2 D2 NULL NULL
3 D3 D4 D6
4 NULL NULL D7
}



Can any one please let me know the query to achieve this

Thank you very much for your time and support

~Moahn