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Using count_distinct on master and detail tables

We have a master and detail table. Can you perform a count_distinct on a column on a master table as well as count_distinct on a detail table?

I'm not exactly sure what you're wanting to do but see if the following does the trick:

SELECT count(distinct o.order_date), count( distinct oi.item_no)
  FROM order_item oi, orders o
 WHERE o.ord_no = oi.ord_no;
In this example, the master table is the ORDERS table and the detail table is ORDER_ITEM. The query selects the distinct count of the order_date column from ORDERS and the distinct count of the item_no column from ORDER_ITEM. The tables are joined on ord_no.

This should work for any non-key column just fine. But if you want to do distinct counts of key columns, ord_no for example, you need to change the query just a bit as follows:

SELECT count(distinct o.ord_no), count( distinct oi.ord_no)
  FROM order_item oi, orders o
 WHERE o.ord_no = oi.ord_no(+) ;
The difference here is the use of the outer join. This way, you would basically always get a valid row count for the ORDERS table even if an order did not have a related detail record (i.e. no items added to the order yet). For example, if the ORDERS table had 10 order records but the ORDER_ITEM table only had items for 7 of the 10 orders, the above statement would return 10 and 7. If you had not used outer join syntax, you would have gotten counts of 7 and 7.

I hope this helps!

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This was last published in May 2002

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