对于关联表的查询使用join或where进行关联.具体的效率有没有很大区别?

Question

两张表.manytoone的关系.查询many,通过外键随便把one端得信息查出.我们可以通过left outer join on鞥等的join查询进行抓取.或者可以通过 where many端得外键id=one端得主键 效果是一样的.那他们具体的查询效率是否有区别?

Answer 1

看情况，如果你的devicespec1.id是主键或者索引什么的，那么join效率高。看这个就清楚了：

mysql> explain select test_tb1.id, test_tb2.value from test_tb1, test_tb2 where test_tb1.id = test_tb2.id;
+----+-------------+----------+-------+---------------+---------+---------+------+------+--------------------------------+
| id | select_type | table    | type  | possible_keys | key     | key_len | ref  | rows | Extra                          |
+----+-------------+----------+-------+---------------+---------+---------+------+------+--------------------------------+
|  1 | SIMPLE      | test_tb1 | index | PRIMARY       | PRIMARY | 4       | NULL |    3 | Using index                    |
|  1 | SIMPLE      | test_tb2 | ALL   | PRIMARY       | NULL    | NULL    | NULL |    4 | Using where; Using join buffer |
+----+-------------+----------+-------+---------------+---------+---------+------+------+--------------------------------+
2 rows in set (0.01 sec)

mysql> explain select test_tb1.id, test_tb2.value from test_tb1 left join test_tb2 on test_tb1.id = test_tb2.id;
+----+-------------+----------+--------+---------------+---------+---------+------------------+------+-------------+
| id | select_type | table    | type   | possible_keys | key     | key_len | ref              | rows | Extra       |
+----+-------------+----------+--------+---------------+---------+---------+------------------+------+-------------+
|  1 | SIMPLE      | test_tb1 | index  | NULL          | PRIMARY | 4       | NULL             |    3 | Using index |
|  1 | SIMPLE      | test_tb2 | eq_ref | PRIMARY       | PRIMARY | 4       | test.test_tb1.id |    1 |             |
+----+-------------+----------+--------+---------------+---------+---------+------------------+------+-------------+
2 rows in set (0.00 sec)

第一次查询要遍历两张表，第二次查询使用eq_ref不用遍历第二张表