left join/inner join what are they

Anon

Probably a simple question. I'm learning SQL at the moment and I was a little confused about what inner and left/right joins actually do - Could someone explain please?

Thanks in advance.

Anon

http://www.mysql.com/doc/J/O/JOIN.html

mhensler

Have you read anything on them?

They can be pretty simple, and they can get very complex (many, many tables). But they sure can be very useful (by cutting down the number of queries to run).

Here is the most used INNER JOIN (mySQL style)...

Lets say you have two tables. 'People' and 'States'. Both tables have an 'ID' field, 'People' has (among other things) a 'StateID' field, and 'States' has a 'Name' field. You want to select everyone living in California.... how do you do it?

You could run two queries... the first getting the 'ID' field from the 'States' table, and then using the value returned query for all rows in the 'People' table WHERE 'StateID' equals your value. Or you could run one query to get relational data from both tables like this:

"SELECT * FROM People p, States s WHERE p.StateID=s.ID AND s.Name='California'"

Anon

You can rewrite that query using a join like this:

SELECT people.* from people
LEFT JOIN states ON people.id=states.id WHERE states.name='California';

Anon

Or even:

SELECT * from people LEFT JOIN states USING(id) WHERE states.name='California';

striker

Does the left join work for databases using odbc commands? I have an access database that i'm trying pull data from two tables and I've been trying to use the following query, but it doesn't seem to be working. I'm getting an error that says too few parameters. Any ideas? thanks!

$sql_statement = "SELECT * FROM abc LEFT JOIN xyz ON
abc.exist_id = xyz.exist_id
WHERE abc.client_id = '$client_id'";