nicht angemeldet
Hi,
danke erst mal für den Tipp. O'Reilly-Bücher kann man sowieso fast wahllos kaufen ;-) aber über SQL in a nutshell bin ich noch nicht gestolpert.
Wenn du was findest was beschreibt wie Subselects möglichst generell gut
umgangen werden können, würde mich dass sehr interessieren.
In der MySQL-Doku finden sich ja einige Ansätze, wie man subselects umgehen kann. Ich umgehe subselects meistens auf Anwendungsebene, was aber in manchen Fällen auch nicht funktioniert. Eine andere Art wären temporäre Tabellen. Hier mal ein Auszug aus der Doku:
--mysql-dokumentation--
1.4.4.1 Sub-selects
MySQL currently only supports sub selects of the form INSERT ... SELECT ... and REPLACE ... SELECT .... You can however use the function IN() in other contexts.
In many cases you can rewrite the query without a sub-select:
SELECT * FROM table1 WHERE id IN (SELECT id FROM table2);
This can be re-written as:
SELECT table1.* FROM table1,table2 WHERE table1.id=table2.id;
The queries:
SELECT * FROM table1 WHERE id NOT IN (SELECT id FROM table2);
SELECT * FROM table1 WHERE NOT EXISTS (SELECT id FROM table2 where table1.id=table2.id);
Can be rewritten as:
SELECT table1.* FROM table1 LEFT JOIN table2 ON table1.id=table2.id where table2.id IS NULL
For more complicated subqueries you can often create temporary tables to hold the subquery. In some cases, however this option will not work. The most frequently encountered of these cases arises with DELETE statements, for which standard SQL does not support joins (except in sub-selects). For this situation there are two options available until subqueries are supported by MySQL.
The first option is to use a procedural programming language (such as Perl or PHP) to submit a SELECT query to obtain the primary keys for the records to be deleted, and then use these values to construct the DELETE statement (DELETE FROM ... WHERE ... IN (key1, key2, ...)).
The second option is to use interactive SQL to contruct a set of DELETE statements automatically, using the MySQL extension CONCAT() (in lieu of the standard || operator). For example:
SELECT CONCAT('DELETE FROM tab1 WHERE pkid = ', tab1.pkid, ';')
FROM tab1, tab2
WHERE tab1.col1 = tab2.col2;
You can place this query in a script file and redirect input from it to the mysql command-line interpreter, piping its output back to a second instance of the interpreter:
prompt> mysql --skip-column-names mydb < myscript.sql | mysql mydb
--ende mysql-dokumentation--
Hoffe, das hilft weiter...
Viele Grüsse,
Achim