DROP COLUMN

In this command, the syntax for dropping columns is as follows:

DROP [COLUMN] column_name

You cannot drop a column if the column is a primary key.

When dropping a column, a considerable amount of preliminary action is required to identify and remove any external dependencies on the column. You must search for any references to the column within triggers, procedures, views, and restriction filters by querying the relevant database tables. You must also search your probe rules files and gateway mapping files for references to the column.

The following guidelines are based on an example scenario where you want to drop the Country column from a table in your ObjectServer:

1. Connect to your ObjectServer (for example, OWL) using the SQL interactive interface, as shown in the following table. Your user name is assumed by default, but you are required to enter your password.

   Table 1. Starting the SQL interactive interface

   Option	Description
   UNIX	Enter: $NCHOME/omnibus/bin/nco_sql -server OWL
   Windows	Enter: %NCHOME%\omnibus\bin\isql -S OWL

2. Back up your ObjectServer to a temporary location (for example, /tmp/mybackup) using the ALTER SYSTEM BACKUP command. This precautionary measure ensures that you can restore your system if required.

   1> alter system backup '/tmp/mybackup';
   2> go

3. List details of your triggers, as stored in the catalog.triggers table:

   1> describe catalog.triggers;
   2> go

   The type of key, name, data type, and length of each column in the table are output to the screen.

4. Retrieve the names of all triggers that reference the Country column in the body or the evaluate clause of the trigger:

   1> select TriggerName from catalog.triggers where CodeBlock like ' Country' or EvaluateBlock like ' Country';

5. Make a note of all listed triggers and remove the Country references by editing each trigger. You can do this from the Trigger Details window (Action tab) in the Netcool/OMNIbus Administrator.
   Important: Before you make any of the changes described in the remaining steps of this procedure, check for the following conditions:

   • There might be two or more tables using the same column name in your schema. If this is the case, identify the correct triggers for the table that you want to drop before you make any changes.
   • Check that there are no dependencies between triggers that might result in unwanted changes.
6. Repeat steps 3 to 5 to identify any other objects that reference the Country column, and to remove all instances of the reference. The following table lists the database tables that you need to search, the relevant SELECT statements, and the Netcool/OMNIbus Administrator windows that you can use to edit the object.

   Table 2. System catalog tables to be searched, SELECT statements, and Netcool/OMNIbus Administrator windows

   Object type	Table name	SELECT statement	Netcool/OMNIbus Administrator window
   Procedures	catalog.sql_procedures	select ProcedureName from catalog.sql_procedures where CodeBlock like 'Country';	SQL Procedure Details window
   Restriction Filters	catalog.restrictions	select RestrictionName from catalog.restrictions where ConditionText like 'Country';	Restriction Filter Details window
   Views	catalog.views	select ViewName from catalog.views where CreationText like 'Country';

7. Search your probe rules files and remove any references to the column.
8. Search your gateway mapping files $NCHOME/omnibus/gates/objserv_type/objserv_type.map, where type represents uni or bi. Remove any references to the column.
9. After all the references have been removed, drop the Country column using the ALTER TABLE ... DROP COLUMN syntax.