CREATE TABLE IF NOT EXISTS `categories` (
`id` int(10) unsigned NOT NULL AUTO_INCREMENT,
`name` varchar(15) NOT NULL,
`created` datetime NOT NULL,
PRIMARY KEY (`id`)
);

CREATE TABLE IF NOT EXISTS `posts` (
`id` int(10) unsigned NOT NULL AUTO_INCREMENT,
`category_id` int(10) unsigned NOT NULL,
`title` varchar(100) NOT NULL,
`content` varchar(200) NOT NULL,
`created` datetime NOT NULL,
PRIMARY KEY (`id`),
KEY `category_id` (`category_id`)
);

EXPLAIN SELECT *
FROM posts
LEFT JOIN categories ON posts.category_id = categories.id
WHERE categories.id = ‘一个已经存在的ID'
ORDER BY posts.created DESC

table   key     Extra
categories PRIMARY   Using filesort
posts   category_id Using where

ALTER TABLE `posts` ADD INDEX ( `category_id` , `created` );

id | name
---------
1 | libk
2 | zyfon
3 | daodao

user_action表:

user_id | action
---------------
1 | jump
1 | kick
1 | jump
2 | run
4 | swim

select id, name, action from user as u
left join user_action a on u.id = a.user_id

result:
id | name | action
--------------------------------
1 | libk | jump ①
1 | libk | kick ②
1 | libk | jump ③
2 | zyfon | run ④
3 | daodao | null ⑤

select id, name, action from user as u
left join user_action a on u.id = a.user_id
where a.user_id is NULL

result:
id | name | action
--------------------------
3 | daodao | NULL

--------------------------------------------------------------------------------

mysql> SELECT * FROM table1,table2 WHERE table1.id=table2.id;
mysql> SELECT * FROM table1 LEFT JOIN table2 ON table1.id=table2.id;
mysql> SELECT * FROM table1 LEFT JOIN table2 USING (id);
mysql> SELECT * FROM table1 LEFT JOIN table2 ON table1.id=table2.id
-> LEFT JOIN table3 ON table2.id=table3.id;
mysql> SELECT * FROM table1 USE INDEX (key1,key2)
-> WHERE key1=1 AND key2=2 AND key3=3;
mysql> SELECT * FROM table1 IGNORE INDEX (key3)
-> WHERE key1=1 AND key2=2 AND key3=3;