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How to limit number of rows in Oracle, using "sample" clause
In Oracle, you can use the "sample" clause in a SELECT statement to specify the number of rows that should be returned in the result set. The "sample" clause is used in the following way:
Code (sql):SELECT column1, column2, ...
FROM my_table
SAMPLE (n)
In this example, the SAMPLE (n) clause specifies that the query should return n rows from the my_table table. The rows that are returned will be chosen randomly from the table.
Here is an example of how you could use the "sample" clause to return a random sample of 10 rows from a table:
Code (sql):SELECT *
FROM my_table
SAMPLE (10);
This query would return 10 random rows from the my_table table. The rows that are returned will be chosen randomly from the table, and the same rows may not be returned if the query is executed multiple times.
I beleive that sample(10) means 10 percent of rows not 10 rows
You are correct, the SAMPLE (n) clause in Oracle SQL specifies that the query should return a random sample of n percent of the rows in a table, rather than a specific number of rows. So if you use the SAMPLE (10) clause in a query, it will return a random sample of 10 percent of the rows in the table.
Here is an example of how you could use the "sample" clause to return a random sample of 10 percent of the rows from a table:
Code:
SELECT *
FROM my_table
SAMPLE (10);
This query would return a random sample of 10 percent of the rows from the my_table table. The rows that are returned will be chosen randomly from the table, and the same rows may not be returned if the query is executed multiple times.
Is there a way to get exactly 10 rows in Oracle SQL language
Yes, you can use the "rownum" keyword in Oracle SQL to limit the number of rows returned in a query to a specific number. The "rownum" keyword is used in the following way:
Code (sql):SELECT column1, column2, ...
FROM my_table
WHERE rownum <= n;
In this example, the WHERE rownum <= n clause specifies that the query should return only the first n rows from the my_table table. The rows that are returned will be the first n rows in the table, based on the order in which they were selected by the query.
Is it possible to use rownum with "order by" clause
Yes, you can use the "rownum" keyword in combination with the "order by" clause in Oracle SQL to limit the number of rows returned in a query to a specific number, and to specify the order in which the rows should be returned. The "order by" clause is used to sort the rows in a query result set based on the values of one or more columns.
Here is an example of how you could use the "rownum" keyword in combination with the "order by" clause to return a specific number of rows from a table, in a specific order:
Code (sql):SELECT *
FROM my_table
WHERE rownum <= 10
ORDER BY column1, column2;
In this example, the WHERE rownum <= 10 clause specifies that the query should return only the first 10 rows from the my_table table. The ORDER BY column1, column2 clause specifies that the rows should be sorted by the values in the column1 and column2 columns. So the query will return the first 10 rows from the table, sorted by the values in the column1 and column2 columns.
But rownum is caluculated before sorting, so query may return less then 10 rows
Yes, you are correct. The "rownum" keyword in Oracle SQL is assigned to each row in a query result set before the rows are sorted by the "order by" clause. So if a query uses both the "rownum" keyword and the "order by" clause, it is possible for the query to return fewer rows than the number specified in the "rownum" clause, because some of the rows may be filtered out by the "order by" clause.
To avoid this problem, you can use a subquery in your query to first apply the "order by" clause, and then use the "rownum" keyword in the outer query to limit the number of rows returned. Here is an example of how you could use a subquery to return exactly 10 rows from a table, in a specific order:
Code (sql):SELECT *
FROM (
SELECT *
FROM my_table
ORDER BY column1, column2
)
WHERE rownum <= 10;
In this example, the subquery applies the "order by" clause to the rows in the my_table table, and then the outer query uses the "rownum" keyword to limit the number of rows returned to 10. This ensures that the query will always return exactly 10 rows, in the specified order.