MySQL Join Explanation [duplicate] - php

Question

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Closed 11 years ago.
Possible Duplicate:
SQL Join Differences
$PERSON = $DATABASE_LINK->query("SELECT * FROM `users`,`profiles` WHERE users.first_name = 'shane' && users.last_name = 'larson' && users.setup = '1' && profiles.zipcode = '53511' ORDER BY `full_name` ASC");
$PERSON = $PERSON->fetch_object();
var_dump($PERSON);
I want a query that scans for a user record based off name, and checks the zip code from the profile table. Above is a example. It works, but idk how joins work exactly. Any explanation on how joins match 2 rows would be awesome :)

Answer 1

the result set that you are receiving is a cross product of all rows in the user table that match the users criteria and all rows in the profiles table that match the profiles criteria.
It seems likely that this is not what you want.
consider adding something like users.blammy = profiles.blammy to join the two tables.

Answer 2

SELECT Cities.Name, Countries.Name, Countries.id
FROM Cities INNER JOIN Countries
ON Cities.CountryId = Countries.id
WHERE Cities.Name LIKE = 'town' LIMIT 10
Looking at that example:
The first row selects three columns from two different tables (Cities and Countries).
The second row specifies the two tables to join.
The third row specifies what item to match the two tables on.
The result from that query is a list
of Cities.Name that match the LIKE
clause and their corresponding
Countries.Name (which it finds by
matching Cities.CountryId with
Countries.id).
Read more here: http://www.wellho.net/mouth/158_MySQL-LEFT-JOIN-and-RIGHT-JOIN-INNER-JOIN-and-OUTER-JOIN.html

Answer 3

So using SQL Statements with "WHERE table1.id_x = table2.id_y" do work, but as far as i know they are slower than Joins (primarly with huge datasets).
So there inner and outer Joins, the outer Joins are divided into left- and right-joins.
A little Explanation for a left-Join:
SELECT Class.Id, Class.Name, Professor.Id, Professor.Name
FROM FROM Professor INNER JOIN Class
ON Professor.Id = Class.ProfessorId;
This Statement Selects all Professors, and does not care if a Professor don't teaches a Class. But if there is a class without a Professor, this class is not selected. It's called left Join because 'NULL' References in the left Table are ignored. The explanation for the right-Join should be trivial ;)
SELECT Class.Id, Class.Name, Professor.Id, Professor.Name
FROM FROM Professor LEFT OUTER JOIN Class
ON Professor.Id = Class.ProfessorId;
The Inner Join don't shows Professors without Classes and it don't shows Classes without Professors.
Please remember: Not all DBMS got the Keywords 'INNER' and 'OUTER' ;)