How write Alter Table and add new columns? - sql-server

Question

I have a table that has 3 columns A,B,C which has rows also. Column A is the primary key.
Now as per new requirement I need to add new column D, E and F.
Also i need to remove the previous primary key from column A and add a new primary key for column D.
Column E and F is NULL.
Please help me to create alter table statement.

Answer 1

What you require is a multi-step process. Adding the columns, dropping the existing primary key constraint and finally adding a new one.
The most difficult thing here is adding column D. Because you want it to be the new primary key it will have to be NOT NULL. If your table has existing data you will need to handle this error:
SQL> alter table your_table
2 add ( d number not null
3 , e date
4 , f number )
5 /
alter table your_table
*
ERROR at line 1:
ORA-01758: table must be empty to add mandatory (NOT NULL) column
SQL>
So, step 1 is add the new columns with D optional; then populate it with whatever key values:
SQL> alter table your_table
2 add ( d number
3 , e date
4 , f number )
5 /
Table altered.
SQL> update your_table
2 set d = rownum
3 /
1 row updated.
SQL>
Now we can make column D mandatory:
SQL> alter table your_table
2 modify d not null
3 /
Table altered.
SQL>
Finally, we can change the primary key column from A to D:
SQL> alter table your_table
2 drop primary key
3 /
Table altered.
SQL> alter table your_table
2 add constraint yt_pk primary key (d)
3 /
Table altered.
SQL>
For some alterations we want to add a column with a default value. In this scenario it is possible to do so in one step:
alter table your_table
add new_col varchar2(1) default 'N' not null;
In later versions of Oracle this is actually an extremely efficient of populating the new column with the same value, considerably faster than the multi-step approach outlined above.
In case it's not clear the above syntax is Oracle. I expect SQL Server will be something similar.