mysql 多表联合查询示例

示例数据库表（鸣谢鞠老师）

mysql> show tables;
+-----------------+
| Tables_in_jol   |
+-----------------+
| compileinfo     |
| contest         |
| contest_problem |
| custominput     |
| loginlog        |
| mail            |
| news            |
| online          |
| privilege       |
| problem         |
| reply           |
| runtimeinfo     |
| sim             |
| solution        |
| source_code     |
| topic           |
| users           |
+-----------------+
17 rows in set (0.00 sec)

mysql> select * from sim;
+------+----------+------+
| s_id | sim_s_id | sim  |
+------+----------+------+
| 5235 |     5229 |   85 |
| 5239 |     5230 |  100 |
| 5243 |     5231 |   85 |
| 5358 |     5339 |  100 |
| 5365 |     5271 |   98 |
| 5383 |     5339 |  100 |
| 5386 |     5339 |  100 |
| 5425 |     5339 |  100 |
| 5514 |     5339 |  100 |
| 5531 |     5518 |  100 |
| 5537 |     5339 |  100 |
| 5539 |     5518 |  100 |
| 5549 |     5339 |  100 |
| 5569 |     5518 |  100 |
| 5601 |     5586 |  100 |
| 5607 |     5602 |   92 |
| 5620 |     5602 |   92 |
| 5674 |     5673 |  100 |
| 5677 |     5466 |  100 |
| 5680 |     5663 |  100 |
+------+----------+------+
20 rows in set (0.00 sec)

mysql> describe solution;
+-------------+-----------------------+------+-----+---------------------+----------------+
| Field       | Type                  | Null | Key | Default             | Extra          |
+-------------+-----------------------+------+-----+---------------------+----------------+
| solution_id | int(11)               | NO   | PRI | NULL                | auto_increment |
| problem_id  | int(11)               | NO   | MUL | 0                   |                |
| user_id     | char(48)              | NO   | MUL | NULL                |                |
| time        | int(11)               | NO   |     | 0                   |                |
| memory      | int(11)               | NO   |     | 0                   |                |
| in_date     | datetime              | NO   |     | 0000-00-00 00:00:00 |                |
| result      | smallint(6)           | NO   | MUL | 0                   |                |
| language    | int(10) unsigned      | NO   |     | 0                   |                |
| ip          | char(15)              | NO   |     | NULL                |                |
| contest_id  | int(11)               | YES  | MUL | NULL                |                |
| valid       | tinyint(4)            | NO   |     | 1                   |                |
| num         | tinyint(4)            | NO   |     | -1                  |                |
| code_length | int(11)               | NO   |     | 0                   |                |
| judgetime   | timestamp             | YES  |     | CURRENT_TIMESTAMP   |                |
| pass_rate   | decimal(2,2) unsigned | NO   |     | 0.00                |                |
| lint_error  | int(10) unsigned      | NO   |     | 0                   |                |
+-------------+-----------------------+------+-----+---------------------+----------------+
16 rows in set (0.00 sec)

mysql> select a1.user_id as "学号(copy)", a2.user_id as "学号(copied)", sim as "相似度(%)" from sim, solution as a1, solution as a2 where s_id=a1.solution_id and sim_s_id=a2.solution_id;

+--------------+----------------+--------------+
| 学号(copy)   | 学号(copied)   | 相似度(%)    |
+--------------+----------------+--------------+
| 2013201410   | heu_songxu     |           85 |
| 2013201410   | heu_songxu     |          100 |
| 2013201410   | heu_songxu     |           85 |
| 2014201104   | 2014201130     |          100 |
| 2014201314   | 2014201321     |           98 |
| 2014201102   | 2014201130     |          100 |
| 2014201101   | 2014201130     |          100 |
| 2014201402   | 2014201130     |          100 |
| 2014201204   | 2014201130     |          100 |
| 2014201213   | 2014201228     |          100 |
| 2014201204   | 2014201130     |          100 |
| 2014201205   | 2014201228     |          100 |
| 2014201106   | 2014201130     |          100 |
| 2014201220   | 2014201228     |          100 |
| 2014201314   | 2014201321     |          100 |
| 2014201213   | 2014201228     |           92 |
| 2014201205   | 2014201228     |           92 |
| 2014201106   | 2014201118     |          100 |
| 2014201417   | 2014201406     |          100 |
| 2014201110   | 2014201221     |          100 |
+--------------+----------------+--------------+
20 rows in set (0.00 sec)