ORDER BY & LIMIT

I have a query that looks like this:

SELECT article FROM table1 ORDER BY publish_date LIMIT 20

How does ORDER BY work? Will it order all records, then get the first 20, or will it get 20 records and order them by the publish_date field?

If it's the last one, you're not guaranteed to really get the most recent 20 articles.

---

它会先排序，然后得到前20个。数据库还会在ORDER BY之前处理WHERE子句中的任何内容。

---

The LIMIT clause can be used to constrain the number of rows returned by the SELECT statement. LIMIT takes one or two numeric arguments, which must both be nonnegative integer constants (except when using prepared statements).

With two arguments, the first argument specifies the offset of the first row to return, and the second specifies the maximum number of rows to return. The offset of the initial row is 0 (not 1):

SELECT * FROM tbl LIMIT 5,10; # Retrieve rows 6-15

To retrieve all rows from a certain offset up to the end of the result set, you can use some large number for the second parameter. This statement retrieves all rows from the 96th row to the last:

SELECT * FROM tbl LIMIT 95,18446744073709551615;

With one argument, the value specifies the number of rows to return from the beginning of the result set:

SELECT * FROM tbl LIMIT 5; # Retrieve first 5 rows

In other words, LIMIT row_count is equivalent to LIMIT 0, row_count.

All details on: http://dev.mysql.com/doc/refman/5.0/en/select.html

---

If there is a suitable index, in this case on the publish_date field, then MySQL need not scan the whole index to get the 20 records requested - the 20 records will be found at the start of the index. But if there is no suitable index, then a full scan of the table will be needed.

There is a MySQL Performance Blog article from 2009 on this.

链接地址: http://www.djcxy.com/p/34270.html

上一篇: Linq排序，按组排列和排序？

下一篇: ORDER BY＆LIMIT