Question

I am trying to join two tables like this…

SELECT * FROM lists l
INNER JOIN products p USING (list_id)

Here is the show create table for both the tables..

CREATE TABLE `lists` (
  `list_id` bigint(20) NOT NULL AUTO_INCREMENT,
  `list_name` varchar(255) NOT NULL,
  `deleted` tinyint(4) NOT NULL DEFAULT '0'
  PRIMARY KEY (`list_id`),
  KEY `index1` (`list_name`),
) ENGINE=InnoDB AUTO_INCREMENT=6 DEFAULT CHARSET=utf8

CREATE TABLE `products` (
  `product_id` bigint(20) NOT NULL AUTO_INCREMENT,
  `list_id` bigint(20) unsigned NOT NULL DEFAULT '0',
  `deleted` tinyint(4) NOT NULL DEFAULT '0',
  PRIMARY KEY (`product_id`),
  KEY `index2` (`deleted`)
) ENGINE=InnoDB AUTO_INCREMENT=11 DEFAULT CHARSET=utf8

Here’s the result of above query:

l.list_id : l.list_name : l.deleted : p.product_id : p.deleted

1   Christmas   0   1   1
1   Christmas   0   2   0
1   Christmas   0   3   0
1   Christmas   0   4   0
4   New Year    0   8   0
4   New Year    0   9   0
4   New Year    0   10  0

You see l.deleted column there with all zeros? In the table lists there are some rows with other values like 1, 2, … in deleted column. But joining the tables always show the deleted column to have all zeros.

Why is this happening? What am I missing here?

Answer 1

I suggest you use a different syntax to express your JOIN. If you use the ON syntax, you’ll have more clarity about what is joining to what. This is stylistic.

Try this:

SELECT * FROM lists l
INNER JOIN products p ON l.list_id = p.list_id

But, I suspect you have items in your lists table (with deleted column values that aren’t zero, that aren’t turning up in your products table. To see those in your result set try this:

SELECT * FROM lists l
LEFT JOIN products p ON l.list_id = p.list_id