开发者

When is it not appropriate to use Derived tables?

This SO post detai开发者_开发知识库ls some benefits in performance regarding Derived vs. Temporary tables.

Other than performance, what situations exist for which a Derived table would not be appropriate.

One answer per post with an example would be helpful.


I would prefer to do a self-join on a temporary table than a derived table.

CREATE TEMPORARY TABLE foo AS SELECT ...;

SELECT ... FROM foo f1 JOIN foo f2 ON ...conditions...;

Versus using a derived table, where you have to write the whole query twice:

SELECT ... 
FROM (SELECT ...)
JOIN (SELECT ...) ON ...conditions...;

But another solution is to use common table expressions which are slightly different from derived tables:

WITH foo AS (SELECT ...)
SELECT ... FROM foo f1 JOIN foo f2 ON ...conditions...;

Provided you use a brand of database that supports this syntax (Microsoft, Oracle, IBM, PostgreSQL).


Scope might be one. I think temp tables can be made accessible to other transactions / processes, etc. Derived tables are limited to the block in which they are declared.


If you have to access the data in the temporary table in multiple queries it might be less costly to avoid generating the temp data repeatedly:

CREATE TEMPORARY TABLE foo AS SELECT ...;

SELECT ... FROM foo WHERE ...conditions...;

-- sometime later

SELECT ... FROM foo WHERE ...different conditions...;
0

上一篇:

下一篇:

精彩评论

暂无评论...
验证码 换一张
取 消

最新问答

问答排行榜