Drop unused fields

Question

Hi
I am using around 130 tables in Oracle, now i want to update all the table and want to drop fields from the table which are not used and having blank record.
going to each table and giving command everytime and identifiying a blank field is time taking job, is there any shortcut for this ???
sanjay

Answer 1

I won't suggest writing a automated script to identify columns not being used and dropping them without even doing a impact-analysis! A better approach would be:
1) Identify columns not being used (say the ones where all rows have null values)
2) Do impact-analysis to short-list the columns that maybe dropped
3) On a table-to-table basis, write scripts to drop the column or mark it as unused, or write create table statements if mutiple columns of the same table need to be dropped. As suggested by Eric above.
Now for step (1), if all the tables of your schema are analyzed, you may use the following script to identify columns that has null value for all the rows:
select *
from   user_tab_columns
where  num_distinct = 0
and    low_value    is null
and    high_value   is null
and    num_nulls    > 0      -- this is to ignore tables with no rows!
and    avg_col_len  = 0
order  by table_name, column_id
          NOTE: Please be aware that I have not tested the above script on a full live-schema. I have tested it with a test schema having 4 tables, all tables having three number columns (col1, col2 and col3) and a test data of 10,000 rows each:
1. Table 1: col3 having null value for all rows.
2. Table 2: col3 having not-null value for all rows.
3. Table 3: col3 having not-null value in only 1 row.
4. Table 4: Table with no rows.