我是一个老派的MySQL用户，总是更喜欢JOIN而不是子查询。但是现在每个人都用子查询，我讨厌它;我不知道为什么。
我缺乏理论知识来判断是否有任何不同。子查询是否与JOIN一样好，因此没有什么可担心的?
当前回答
MySQL版本:5.5.28-0ubuntu0.12.04.2-log
在我的印象中，JOIN总是比MySQL中的子查询更好，但EXPLAIN是更好的判断方式。下面是一个子查询比join更好的例子。
这是我的查询与3个子查询:
EXPLAIN SELECT vrl.list_id,vrl.ontology_id,vrl.position,l.name AS list_name, vrlih.position AS previous_position, vrl.moved_date
FROM `vote-ranked-listory` vrl
INNER JOIN lists l ON l.list_id = vrl.list_id
INNER JOIN `vote-ranked-list-item-history` vrlih ON vrl.list_id = vrlih.list_id AND vrl.ontology_id=vrlih.ontology_id AND vrlih.type='PREVIOUS_POSITION'
INNER JOIN list_burial_state lbs ON lbs.list_id = vrl.list_id AND lbs.burial_score < 0.5
WHERE vrl.position <= 15 AND l.status='ACTIVE' AND l.is_public=1 AND vrl.ontology_id < 1000000000
AND (SELECT list_id FROM list_tag WHERE list_id=l.list_id AND tag_id=43) IS NULL
AND (SELECT list_id FROM list_tag WHERE list_id=l.list_id AND tag_id=55) IS NULL
AND (SELECT list_id FROM list_tag WHERE list_id=l.list_id AND tag_id=246403) IS NOT NULL
ORDER BY vrl.moved_date DESC LIMIT 200;
解释说明:
+----+--------------------+----------+--------+-----------------------------------------------------+--------------+---------+-------------------------------------------------+------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+--------------------+----------+--------+-----------------------------------------------------+--------------+---------+-------------------------------------------------+------+--------------------------+
| 1 | PRIMARY | vrl | index | PRIMARY | moved_date | 8 | NULL | 200 | Using where |
| 1 | PRIMARY | l | eq_ref | PRIMARY,status,ispublic,idx_lookup,is_public_status | PRIMARY | 4 | ranker.vrl.list_id | 1 | Using where |
| 1 | PRIMARY | vrlih | eq_ref | PRIMARY | PRIMARY | 9 | ranker.vrl.list_id,ranker.vrl.ontology_id,const | 1 | Using where |
| 1 | PRIMARY | lbs | eq_ref | PRIMARY,idx_list_burial_state,burial_score | PRIMARY | 4 | ranker.vrl.list_id | 1 | Using where |
| 4 | DEPENDENT SUBQUERY | list_tag | ref | list_tag_key,list_id,tag_id | list_tag_key | 9 | ranker.l.list_id,const | 1 | Using where; Using index |
| 3 | DEPENDENT SUBQUERY | list_tag | ref | list_tag_key,list_id,tag_id | list_tag_key | 9 | ranker.l.list_id,const | 1 | Using where; Using index |
| 2 | DEPENDENT SUBQUERY | list_tag | ref | list_tag_key,list_id,tag_id | list_tag_key | 9 | ranker.l.list_id,const | 1 | Using where; Using index |
+----+--------------------+----------+--------+-----------------------------------------------------+--------------+---------+-------------------------------------------------+------+--------------------------+
使用join的相同查询是:
EXPLAIN SELECT vrl.list_id,vrl.ontology_id,vrl.position,l.name AS list_name, vrlih.position AS previous_position, vrl.moved_date
FROM `vote-ranked-listory` vrl
INNER JOIN lists l ON l.list_id = vrl.list_id
INNER JOIN `vote-ranked-list-item-history` vrlih ON vrl.list_id = vrlih.list_id AND vrl.ontology_id=vrlih.ontology_id AND vrlih.type='PREVIOUS_POSITION'
INNER JOIN list_burial_state lbs ON lbs.list_id = vrl.list_id AND lbs.burial_score < 0.5
LEFT JOIN list_tag lt1 ON lt1.list_id = vrl.list_id AND lt1.tag_id = 43
LEFT JOIN list_tag lt2 ON lt2.list_id = vrl.list_id AND lt2.tag_id = 55
INNER JOIN list_tag lt3 ON lt3.list_id = vrl.list_id AND lt3.tag_id = 246403
WHERE vrl.position <= 15 AND l.status='ACTIVE' AND l.is_public=1 AND vrl.ontology_id < 1000000000
AND lt1.list_id IS NULL AND lt2.tag_id IS NULL
ORDER BY vrl.moved_date DESC LIMIT 200;
输出为:
+----+-------------+-------+--------+-----------------------------------------------------+--------------+---------+---------------------------------------------+------+----------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+--------+-----------------------------------------------------+--------------+---------+---------------------------------------------+------+----------------------------------------------+
| 1 | SIMPLE | lt3 | ref | list_tag_key,list_id,tag_id | tag_id | 5 | const | 2386 | Using where; Using temporary; Using filesort |
| 1 | SIMPLE | l | eq_ref | PRIMARY,status,ispublic,idx_lookup,is_public_status | PRIMARY | 4 | ranker.lt3.list_id | 1 | Using where |
| 1 | SIMPLE | vrlih | ref | PRIMARY | PRIMARY | 4 | ranker.lt3.list_id | 103 | Using where |
| 1 | SIMPLE | vrl | ref | PRIMARY | PRIMARY | 8 | ranker.lt3.list_id,ranker.vrlih.ontology_id | 65 | Using where |
| 1 | SIMPLE | lt1 | ref | list_tag_key,list_id,tag_id | list_tag_key | 9 | ranker.lt3.list_id,const | 1 | Using where; Using index; Not exists |
| 1 | SIMPLE | lbs | eq_ref | PRIMARY,idx_list_burial_state,burial_score | PRIMARY | 4 | ranker.vrl.list_id | 1 | Using where |
| 1 | SIMPLE | lt2 | ref | list_tag_key,list_id,tag_id | list_tag_key | 9 | ranker.lt3.list_id,const | 1 | Using where; Using index |
+----+-------------+-------+--------+-----------------------------------------------------+--------------+---------+---------------------------------------------+------+----------------------------------------------+
rows列的比较表明了差异，使用join的查询使用的是using temporary;使用filesort。
当然，当我运行这两个查询时，第一个查询在0.02秒内完成，第二个查询甚至在1分钟后都没有完成，所以EXPLAIN正确地解释了这些查询。
如果我在list_tag表上没有INNER JOIN，即如果我删除
AND (SELECT list_id FROM list_tag WHERE list_id=l.list_id AND tag_id=246403) IS NOT NULL
从第一个查询和相应的:
INNER JOIN list_tag lt3 ON lt3.list_id = vrl.list_id AND lt3.tag_id = 246403
从第二个查询开始，那么EXPLAIN为两个查询返回相同的行数，并且这两个查询的运行速度相同。
其他回答
使用EXPLAIN查看数据库如何对数据执行查询。这个答案中有一个很大的“视情况而定”……
PostgreSQL可以将子查询重写为连接，或将连接重写为子查询，如果它认为其中一个比另一个快。这完全取决于数据、索引、相关性、数据量、查询等。
在大多数情况下，join比子查询快，子查询比子查询快的情况非常罕见。
在join中，RDBMS可以为您的查询创建一个更好的执行计划，并可以预测应该加载哪些数据来处理并节省时间，不像子查询，它将运行所有的查询并加载所有的数据来进行处理。
子查询的好处是它们比join更可读:这就是为什么大多数新学习SQL的人更喜欢它们;这是简单的方法;但是当涉及到性能时，join在大多数情况下更好，尽管它们也不难读。
MySQL版本:5.5.28-0ubuntu0.12.04.2-log
在我的印象中，JOIN总是比MySQL中的子查询更好，但EXPLAIN是更好的判断方式。下面是一个子查询比join更好的例子。
这是我的查询与3个子查询:
EXPLAIN SELECT vrl.list_id,vrl.ontology_id,vrl.position,l.name AS list_name, vrlih.position AS previous_position, vrl.moved_date
FROM `vote-ranked-listory` vrl
INNER JOIN lists l ON l.list_id = vrl.list_id
INNER JOIN `vote-ranked-list-item-history` vrlih ON vrl.list_id = vrlih.list_id AND vrl.ontology_id=vrlih.ontology_id AND vrlih.type='PREVIOUS_POSITION'
INNER JOIN list_burial_state lbs ON lbs.list_id = vrl.list_id AND lbs.burial_score < 0.5
WHERE vrl.position <= 15 AND l.status='ACTIVE' AND l.is_public=1 AND vrl.ontology_id < 1000000000
AND (SELECT list_id FROM list_tag WHERE list_id=l.list_id AND tag_id=43) IS NULL
AND (SELECT list_id FROM list_tag WHERE list_id=l.list_id AND tag_id=55) IS NULL
AND (SELECT list_id FROM list_tag WHERE list_id=l.list_id AND tag_id=246403) IS NOT NULL
ORDER BY vrl.moved_date DESC LIMIT 200;
解释说明:
+----+--------------------+----------+--------+-----------------------------------------------------+--------------+---------+-------------------------------------------------+------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+--------------------+----------+--------+-----------------------------------------------------+--------------+---------+-------------------------------------------------+------+--------------------------+
| 1 | PRIMARY | vrl | index | PRIMARY | moved_date | 8 | NULL | 200 | Using where |
| 1 | PRIMARY | l | eq_ref | PRIMARY,status,ispublic,idx_lookup,is_public_status | PRIMARY | 4 | ranker.vrl.list_id | 1 | Using where |
| 1 | PRIMARY | vrlih | eq_ref | PRIMARY | PRIMARY | 9 | ranker.vrl.list_id,ranker.vrl.ontology_id,const | 1 | Using where |
| 1 | PRIMARY | lbs | eq_ref | PRIMARY,idx_list_burial_state,burial_score | PRIMARY | 4 | ranker.vrl.list_id | 1 | Using where |
| 4 | DEPENDENT SUBQUERY | list_tag | ref | list_tag_key,list_id,tag_id | list_tag_key | 9 | ranker.l.list_id,const | 1 | Using where; Using index |
| 3 | DEPENDENT SUBQUERY | list_tag | ref | list_tag_key,list_id,tag_id | list_tag_key | 9 | ranker.l.list_id,const | 1 | Using where; Using index |
| 2 | DEPENDENT SUBQUERY | list_tag | ref | list_tag_key,list_id,tag_id | list_tag_key | 9 | ranker.l.list_id,const | 1 | Using where; Using index |
+----+--------------------+----------+--------+-----------------------------------------------------+--------------+---------+-------------------------------------------------+------+--------------------------+
使用join的相同查询是:
EXPLAIN SELECT vrl.list_id,vrl.ontology_id,vrl.position,l.name AS list_name, vrlih.position AS previous_position, vrl.moved_date
FROM `vote-ranked-listory` vrl
INNER JOIN lists l ON l.list_id = vrl.list_id
INNER JOIN `vote-ranked-list-item-history` vrlih ON vrl.list_id = vrlih.list_id AND vrl.ontology_id=vrlih.ontology_id AND vrlih.type='PREVIOUS_POSITION'
INNER JOIN list_burial_state lbs ON lbs.list_id = vrl.list_id AND lbs.burial_score < 0.5
LEFT JOIN list_tag lt1 ON lt1.list_id = vrl.list_id AND lt1.tag_id = 43
LEFT JOIN list_tag lt2 ON lt2.list_id = vrl.list_id AND lt2.tag_id = 55
INNER JOIN list_tag lt3 ON lt3.list_id = vrl.list_id AND lt3.tag_id = 246403
WHERE vrl.position <= 15 AND l.status='ACTIVE' AND l.is_public=1 AND vrl.ontology_id < 1000000000
AND lt1.list_id IS NULL AND lt2.tag_id IS NULL
ORDER BY vrl.moved_date DESC LIMIT 200;
输出为:
+----+-------------+-------+--------+-----------------------------------------------------+--------------+---------+---------------------------------------------+------+----------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+--------+-----------------------------------------------------+--------------+---------+---------------------------------------------+------+----------------------------------------------+
| 1 | SIMPLE | lt3 | ref | list_tag_key,list_id,tag_id | tag_id | 5 | const | 2386 | Using where; Using temporary; Using filesort |
| 1 | SIMPLE | l | eq_ref | PRIMARY,status,ispublic,idx_lookup,is_public_status | PRIMARY | 4 | ranker.lt3.list_id | 1 | Using where |
| 1 | SIMPLE | vrlih | ref | PRIMARY | PRIMARY | 4 | ranker.lt3.list_id | 103 | Using where |
| 1 | SIMPLE | vrl | ref | PRIMARY | PRIMARY | 8 | ranker.lt3.list_id,ranker.vrlih.ontology_id | 65 | Using where |
| 1 | SIMPLE | lt1 | ref | list_tag_key,list_id,tag_id | list_tag_key | 9 | ranker.lt3.list_id,const | 1 | Using where; Using index; Not exists |
| 1 | SIMPLE | lbs | eq_ref | PRIMARY,idx_list_burial_state,burial_score | PRIMARY | 4 | ranker.vrl.list_id | 1 | Using where |
| 1 | SIMPLE | lt2 | ref | list_tag_key,list_id,tag_id | list_tag_key | 9 | ranker.lt3.list_id,const | 1 | Using where; Using index |
+----+-------------+-------+--------+-----------------------------------------------------+--------------+---------+---------------------------------------------+------+----------------------------------------------+
rows列的比较表明了差异，使用join的查询使用的是using temporary;使用filesort。
当然，当我运行这两个查询时，第一个查询在0.02秒内完成，第二个查询甚至在1分钟后都没有完成，所以EXPLAIN正确地解释了这些查询。
如果我在list_tag表上没有INNER JOIN，即如果我删除
AND (SELECT list_id FROM list_tag WHERE list_id=l.list_id AND tag_id=246403) IS NOT NULL
从第一个查询和相应的:
INNER JOIN list_tag lt3 ON lt3.list_id = vrl.list_id AND lt3.tag_id = 246403
从第二个查询开始，那么EXPLAIN为两个查询返回相同的行数，并且这两个查询的运行速度相同。
子查询通常用于将单行作为原子值返回，不过它们也可以用于用IN关键字比较多行之间的值。在SQL语句中几乎任何有意义的地方都允许使用它们，包括目标列表、WHERE子句等等。一个简单的子查询可以用作搜索条件。例如，在一对表之间:
SELECT title
FROM books
WHERE author_id = (
SELECT id
FROM authors
WHERE last_name = 'Bar' AND first_name = 'Foo'
);
注意，在子查询的结果上使用普通值操作符要求只返回一个字段。如果你想检查一个值是否存在于一组其他值中，请使用in:
SELECT title
FROM books
WHERE author_id IN (
SELECT id FROM authors WHERE last_name ~ '^[A-E]'
);
这显然不同于LEFT-JOIN，你只是想连接表a和表B的东西，即使连接条件在表B中没有找到任何匹配的记录，等等。
如果你只是担心速度，你必须检查你的数据库，写一个好的查询，看看是否有显著的性能差异。
首先，为了比较这两个，首先你应该区分查询和子查询:
一个子查询类，它总是使用连接编写相应的等效查询 不能使用连接重写的子查询类
对于第一类查询，一个好的RDBMS将把联接查询和子查询视为等效的，并将产生相同的查询计划。
现在甚至mysql也这么做了。
尽管如此，有时它并不会，但这并不意味着连接总是会赢-我有在mysql中使用子查询提高性能的情况。(例如，如果有一些东西阻止mysql计划器正确估计成本，如果计划器没有看到连接变量和子查询变量相同，那么子查询可以通过强制某个路径来优于连接)。
结论是，如果您想确定哪一种查询性能更好，就应该同时测试连接和子查询变量。
对于第二个类，比较没有意义，因为这些查询不能使用连接重写，在这种情况下，子查询是完成所需任务的自然方式，您不应该歧视它们。
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