What is rownum in oracle

ROWNUM in Oracle is a pseudo column that represents the row sequence number in the current query result. It is mainly used for paging queries, row number display and avoiding duplicate data.

ROWNUM in Oracle

meaning

ROWNUM is Oracle A pseudo column in which represents the sequence number of rows in the current query results.

Syntax

ROWNUM is usually used with the ORDER BY clause, the syntax is as follows:

<code class="sql">SELECT column_list
FROM table_name
ORDER BY column_name
ROWNUM <= n</code>

Where:

• column_list is the data column to be extracted
• table_name is the target table
• column_name is the column to be sorted by
• n is the number of rows to be limited

Purpose

ROWNUM is mainly used for the following purposes:

• Paging query: By limiting the value of ROWNUM, paging query can be implemented, so that only a part of the data can be retrieved at a time.
• Row number display: Display the sequence number of the row in the query results.
• Avoid duplicate data: In some cases, ROWNUM can help avoid duplicate data in query results.

Example

Paging query:

<code class="sql">SELECT *
FROM employees
ORDER BY employee_id
ROWNUM <= 10</code>

Line number display:

<code class="sql">SELECT ROWNUM AS row_number, *
FROM employees
ORDER BY employee_id</code>

Avoid duplicate data:

<code class="sql">SELECT DISTINCT employee_name
FROM employees
WHERE ROWNUM = 1</code>

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