Paging in oracle is a technique for retrieving database result sets, reducing network transmission and memory consumption by splitting the result set into smaller chunks, using the "ROWNUM" keyword and subquery, program Developers can easily implement paging functionality and select specific blocks of data to display based on user needs.
The operating environment of this tutorial: Windows 10 system, Oracle version 19c, DELL G3 computer.
In Oracle, paging is a technique used to retrieve database result sets. It allows programmers to display large amounts of data on multiple pages, segmented and displayed as per the user's needs.
In traditional database queries, when a large amount of data needs to be retrieved, the database will return all results to the client. This can cause network latency and memory consumption issues, especially when the result set is very large. In order to solve this problem, paging technology came into being.
Paging allows programmers to split the result set into smaller chunks, returning only one page of data at a time. This reduces the amount of data transferred over the network and allows for better management of memory consumption.
In Oracle, paging is usually implemented using the ROWNUM keyword and subquery. ROWNUM is an Oracle-specific pseudo-column that assigns a unique number to each returned row. By using ROWNUM, programmers can limit the size of the result set and select the chunks of data returned.
Here is a sample query that demonstrates how to do paging in Oracle:
SELECT *
FROM (
SELECT t.*, ROWNUM AS rn
FROM (
SELECT *
FROM your_table
ORDER BY your_column
) t
WHERE ROWNUM <= :end_row
)
WHERE rn >= :start_row;In this query, we first sort the original result set (ORDER BY your_column). We then process the query results as a subquery(t) and assign each row a ROWNUM value. In the outer query, we use the WHERE clause to limit the number of rows returned and select specific data blocks by setting the start row (:start_row) and end row (:end_row).
By adjusting the start row and end row values, programmers can display different data blocks on different pages. For example, if each page displays 10 rows of data, the starting and ending rows of the first page are 1 and 10, respectively, the second page is 11 and 20, and so on.
It should be noted that ROWNUM in Oracle is allocated after the row is selected, so we need to use nested subqueries to correctly implement the paging function.
In addition to ROWNUM, Oracle also provides some other functions and keywords for paging, such as ROW_NUMBER, OFFSET and FETCH FIRST. These functions can be selected and used according to specific needs.
In summary, paging in Oracle is a technique used to retrieve and display large amounts of data. It reduces network transfer and memory consumption by splitting the result set into smaller chunks. Using the ROWNUM keyword and subqueries, programmers can easily implement paging functionality and select specific data blocks for display based on user needs.
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