mysql left join question - mysql

Question

I want make query select all names from table 'a' where from table 'b' i have id_one='3'. id_two is id record from table
'a', two records have relation from id_one='3'. How i can make query ?
CREATE TABLE IF NOT EXISTS `a` (
`id` int(11) NOT NULL,
`name` varchar(11) NOT NULL,
`value` int(11) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
--
-- Zrzut danych tabeli `a`
--
INSERT INTO `a` (`id`, `name`, `value`) VALUES
(1, 'lalala', 0),
(2, 'allalala', 0);
CREATE TABLE IF NOT EXISTS `b` (
`id_one` int(11) NOT NULL,
`id_two` int(11) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
--
-- Zrzut danych tabeli `b`
--
INSERT INTO `b` (`id_one`, `id_two`) VALUES
(3, 1),
(3, 2);

Answer 1

This is what you want:
select Name
from a inner join b on a.id = b.id_two
where b.id_one = 3

Answer 2

sorry I'm not fully understanding your schemas or your question, but I think what you're trying to ask for is:
SELECT * FROM a
JOIN b on a.id = b.id_two;
Try that.

Answer 3

Your question title mentions left joins, but you don't need a left join to make the query you described.
Left joins are good for finding things that don't match up the way you'd expect. So, using a left join in this case depends on what you are looking for. If you are looking for b.id_two entries that don't have corresponding table a entries,
select Name, b.* from b left join a on a.id = b.id_two
This will give you a table that lists every row in table b, with NULLs in place of the names for table a where there is no match.
Likewise, if you are looking for names that don't have entries in b.id_two, you would use
select Name, b.* from a left join b on a.id = b.id_two
If you want to enforce that there is always a correspondence, you can define a foreign key constraint between the parent and child table.

Answer 4

select Name
from a join b on a.id = b.id_two
where b.id_one = 3;
Will also work to get your answer. Might I also suggest you significantly improve your create table statements to include indexing. E.G.
CREATE TABLE IF NOT EXISTS `a` (
`id` bigint(20) NOT NULL AUTO_INCREMENT,
`name` varchar(11) NOT NULL,
`value` int(11) NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
CREATE TABLE IF NOT EXISTS `b` (
`id_one` bigint(20) NOT NULL,
`id_two` bigint(20) NOT NULL,
KEY `FKCAFBB09382DEAC` (`id_one`),
CONSTRAINT `b_a_1` FOREIGN KEY (`id_two`) REFERENCES `a` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
I would also use bigint for primary keys and use charset=utf8
these days it is just to common to want to migrate application to multi-lingual, lay the ground work now. IMHO