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:
SELECT column_list FROM table_name ORDER BY column_name ROWNUM <= n
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:
SELECT * FROM employees ORDER BY employee_id ROWNUM <= 10
Line number display:
SELECT ROWNUM AS row_number, * FROM employees ORDER BY employee_id
Avoid duplicate data:
SELECT DISTINCT employee_name FROM employees WHERE ROWNUM = 1
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