How to write oracle paging

Oracle paging is a very common requirement, especially when developing web applications and APIs. Pagination can effectively reduce server resource usage and provide users with a more friendly experience. So, how should Oracle paging be written? Let’s introduce it below.

First of all, we need to clarify our needs. Suppose we have an order table named orders, which contains a lot of order information. We need to query the specified page number data according to a fixed number of items per page. For example, each page displays 10 items and queries the order data on page 3. So, how to achieve it? Next, we will introduce several implementation methods in detail.

1. Use subquery to implement paging

Subquery is a common way to implement paging in Oracle, which can be achieved by using ROWNUM. The basic idea is to use ROWNUM in the inner query for sorting, and then use ROWNUM in the outer query for paging operations. The code is as follows:

SELECT * FROM (
  SELECT orders.*, ROWNUM rnum
  FROM (
    SELECT * FROM orders
    ORDER BY order_date DESC
  ) orders
  WHERE ROWNUM <= 30
) WHERE rnum >= 21;

In the above code, our query statement contains three levels of nesting. The outermost SELECT statement uses the WHERE clause to filter ROWNUM and implement paging operations. The inner query statement contains an ORDER BY statement, which is mainly used to sort data. The innermost SELECT statement specifies the table orders we want to query.

2. Use the ROW_NUMBER() function to implement paging

Another way to implement paging is to use the ROW_NUMBER() function. This method is very useful when we need to sort by a specified column. The code is as follows:

SELECT *
FROM (
  SELECT orders.*,
         ROW_NUMBER() OVER(ORDER BY order_date DESC) AS row_num
  FROM orders
) WHERE row_num BETWEEN 21 AND 30;

In the above code, we use the ROW_NUMBER() function to generate a new field row_num, which is used to specify the position of each record after sorting. We can use the WHERE clause to filter records within a specified range.

3. Use OFFSET-FETCH to implement paging

After Oracle 12c, a new way to implement paging was introduced, which is to use the OFFSET-FETCH statement. Compared with the previous two methods, this method is more intuitive and concise, and the code is easier to read and understand. The code is as follows:

SELECT *
FROM orders
ORDER BY order_date DESC
OFFSET 20 ROWS FETCH NEXT 10 ROWS ONLY;

In the above code, we use the OFFSET-FETCH statement to implement paging operations. OFFSET is used to specify the offset, that is, which row to start querying from; FETCH is used to specify the number of rows to query. This method can greatly simplify query statements and improve query efficiency.

To sum up, this article introduces three ways to implement Oracle paging, including using subquery, ROW_NUMBER() function and OFFSET-FETCH statement. Each method has its applicable scenarios, and the specific implementation methods are also slightly different. We can choose the corresponding method to implement Oracle paging according to different needs.

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