如何在Oracle中获取多列的最大值

Question

Here is a sample table:

|           customer_token             |        created_date          | orders | views |
+--------------------------------------+------------------------------+--------+-------+
| 93a03e36-83a0-494b-bd68-495f54f406ca | 10-NOV-14 14.41.09.000000000 |    1   |   0   |
| 93a03e36-83a0-494b-bd68-495f54f406ca | 20-NOV-14 14.41.47.000000000 |    0   |   1   |
| 93a03e36-83a0-494b-bd68-495f54f406ca | 26-OCT-14 16.14.30.000000000 |    2   |   0   |
| 93a03e36-83a0-494b-bd68-495f54f406ca | 11-OCT-14 16.31.11.000000000 |    0   |   2   |

In this customer data table I store all of the dates when a given customer has placed an order, or viewed a product. Now, for a report, I want to write a query where for each customer (auth_token), I want to generate the last_order_date (row where orders > 0) and last_view_date (row where product_views > 0).

I am looking for an efficient query as I have millions of records.

Answer 1

select customer_token, 
       max(case when orders > 0 then created_date else NULL end),
       max(case when views > 0 then created_date else NULL end)
from Customer
group by customer_token;

Update : This query is quite efficient because Oracle is likely to scan the table only once. Also there is an interesting thing with grouping - when you use GROUP BY a select list can only contain columns which are in the GROUP BY or aggregate functions. In this query MAX is calculated for the column created_date , but you don't need to put orders and views in a GROUP BY because they are in the expression inside MAX function. It's not very common.

Answer 2

When you want to get the largest value from a row, you need to use the MAX() aggregate function. It is also best practice to group a column when you are using aggregate functions.

In this case, you want to group by customer_token. That way, you'll receive one row per group, and the aggregate function will give you the value for that group.

However, you only want to see the dates where the cell value is greater than 0, so I recommend you put a case statement inside your MAX() function like this:

SELECT customer_token, 
   MAX(CASE WHEN orders > 0 THEN created_date ELSE NULL END) AS latestOrderDate, 
   MAX(CASE WHEN views > 0 THEN created_date ELSE NULL END) AS latestViewDate
FROM customer
GROUP BY customer_token;

This will give you the max date only when orders is positive, and only when views is positive. Without that case statement, the DBMS won't know which groups to give you, and you would likely get incorrect results.

Here is an oracle reference for aggregate functions .