Обсуждение: join syntax
One of my colleagues has created a database where he has the same field name in two tables and uses this field to link histables rather than some arbitrary value. For example, he has used "exp_id" in two tables. When writing his joins he usesa syntax that says something like JOIN ON EXP_ID. Can someone tell me what that syntax should be? I am not very familiarwith it since I typically use the syntax where one field is set equal to the other.<br /> Personally I prefer notto set databases up this way but cannot seem to convince him of this. And yet I am supposed to now help him with his databaseand application.. Is there some documentation that would define this type of syntax? How is this handled if you havemore than one table in the join? It does not appear that this format would allow for this.<br /> Thanks<br /> Jodi<br/><div class="moz-signature">-- <br /></div><div class="Section1"><p class="MsoNormal"><i><span style="font-size:9pt; font-family: Arial; "> _______________________________<br /></span></i><i><span style="font-size: 10pt;">Jodi L Kanter<br /> BioInformatics Database Administrator<br /> University of Virginia<br /> (434) 924-2846<br /><ahref="mailto:jkanter@virginia.edu">jkanter@virginia.edu</a> </span></i><span style="font-size: 11pt; font-family: Arial;"><br style="mso-special-character:line-break" /><br style="mso-special-character:line-break" /></span><p class="MsoNormal"><spanstyle="font-size: 11pt; font-family: Arial; "> </span><p class="MsoNormal"><i><span style="font-size:9pt; font-family: Arial; "> </span></i><p class="MsoNormal"><i><span style="font-size: 9pt; font-family:Arial; "> </span></i></div>
Jodi, > One of my colleagues has created a database where he has the same field > name in two tables and uses this field to link his tables rather than > some arbitrary value. For example, he has used "exp_id" in two tables. > When writing his joins he uses a syntax that says something like JOIN ON > EXP_ID. Can someone tell me what that syntax should be? I am not very > familiar with it since I typically use the syntax where one field is set > equal to the other. table_a JOIN table_b USING (exp_id) > Personally I prefer not to set databases up this way but cannot seem to > convince him of this. And yet I am supposed to now help him with his > database and application.. That, we can't help you with. Point out to your friend, though, that an explicit JOIN order can often harm performance when the JOIN order given by the query writer is not ideal. That is, the "FROM table, table, table WHERE expression, expression" syntax gives the parser a freer hand to choose the fastest execution method. Of course, on a very small database that typically makes litte difference. -- -Josh BerkusAglio Database SolutionsSan Francisco
On Tue, 17 Jun 2003, Jodi Kanter wrote: > One of my colleagues has created a database where he has the same field > name in two tables and uses this field to link his tables rather than > some arbitrary value. For example, he has used "exp_id" in two tables. > When writing his joins he uses a syntax that says something like JOIN ON > EXP_ID. Can someone tell me what that syntax should be? I am not very > familiar with it since I typically use the syntax where one field is set > equal to the other. > Personally I prefer not to set databases up this way but cannot seem to > convince him of this. And yet I am supposed to now help him with his > database and application.. Is there some documentation that would define > this type of syntax? How is this handled if you have more than one table > in the join? It does not appear that this format would allow for this. If you're joining two tables on a field that has the same name you can use a natural join. select * from table1 natural join table2;
how can i compare two querys' eficiency??? TIA, -- Lucas Lain lainl@aconectarse.com #! /Scripting/Manager (??)
Take a look at EXPLAIN and EXPLAIN ANALYSE: http://www.postgresql.org/docs/view.php?version=7.3&idoc=1&file=sql-explain.html On Wed, 2003-06-18 at 16:23, Lucas Lain wrote: > how can i compare two querys' eficiency??? > > TIA, -- Rod Taylor <rbt@rbt.ca> PGP Key: http://www.rbt.ca/rbtpub.asc