select * from a,b探讨

select * from a,b探讨

今天看同事代码里使用了select * from a,b where a.id=b.id，而我平时都是使用select * from a inner join b where a.id=b.id，于是查了下，发现：

1）单纯的select * from a,b是笛卡尔乘积

2）select * from a,b where a.id=b.id相当于inner join

验证

1）创建两张表

create table userinfo(
        uid int(10) not null default 0,
        report_id int(10) not null default 0,
        primary key(uid)
       ) engine=innodb default charset=utf8;

create table report(
       report_id int(10) not null default 0,
       description varchar(255) default '',
       primary key(report_id)
       ) engine=innodb default charset=utf8;

2）插入测试数据

insert into userinfo values(1,1),(2,1),(3,2),(4,6);
insert into report values(1,'第一条'),(2,'第二条'),(3,'第三条');

mysql> select * from userinfo;
+-----+-----------+
| uid | report_id |
+-----+-----------+
|   1 |         1 |
|   2 |         1 |
|   3 |         2 |
|   4 |         6 |
+-----+-----------+
4 rows in set (0.00 sec)

mysql> select * from report;
+-----------+-------------+
| report_id | description |
+-----------+-------------+
|         1 | 第一条      |
|         2 | 第二条      |
|         3 | 第三条      |
+-----------+-------------+
3 rows in set (0.00 sec)

3）验证

单独的select * from a,b

select * from userinfo,report

结果

mysql> select * from userinfo,report;
+-----+-----------+-----------+-------------+
| uid | report_id | report_id | description |
+-----+-----------+-----------+-------------+
|   1 |         1 |         1 | 第一条      |
|   1 |         1 |         2 | 第二条      |
|   1 |         1 |         3 | 第三条      |
|   2 |         1 |         1 | 第一条      |
|   2 |         1 |         2 | 第二条      |
|   2 |         1 |         3 | 第三条      |
|   3 |         2 |         1 | 第一条      |
|   3 |         2 |         2 | 第二条      |
|   3 |         2 |         3 | 第三条      |
|   4 |         6 |         1 | 第一条      |
|   4 |         6 |         2 | 第二条      |
|   4 |         6 |         3 | 第三条      |
+-----+-----------+-----------+-------------+
12 rows in set (0.00 sec)

可见select * from a,b是笛卡儿积

再来验证select * from a,b where a.id=b.id

mysql> select * from userinfo,report where userinfo.report_id=report.report_id;
+-----+-----------+-----------+-------------+
| uid | report_id | report_id | description |
+-----+-----------+-----------+-------------+
|   1 |         1 |         1 | 第一条      |
|   2 |         1 |         1 | 第一条      |
|   3 |         2 |         2 | 第二条      |
+-----+-----------+-----------+-------------+
3 rows in set (0.00 sec)

inner join

mysql> select * from userinfo inner join report where userinfo.report_id=report.report_id;
+-----+-----------+-----------+-------------+
| uid | report_id | report_id | description |
+-----+-----------+-----------+-------------+
|   1 |         1 |         1 | 第一条      |
|   2 |         1 |         1 | 第一条      |
|   3 |         2 |         2 | 第二条      |
+-----+-----------+-----------+-------------+
3 rows in set (0.00 sec)

mysql> select * from userinfo inner join report on userinfo.report_id=report.report_id;
+-----+-----------+-----------+-------------+
| uid | report_id | report_id | description |
+-----+-----------+-----------+-------------+
|   1 |         1 |         1 | 第一条      |
|   2 |         1 |         1 | 第一条      |
|   3 |         2 |         2 | 第二条      |
+-----+-----------+-----------+-------------+
3 rows in set (0.00 sec)

可见是select * from a,b where a.id=b.id只是把笛卡尔积做了一层过滤，结果与inner join相同

补充：inner join是先生成一个临时表，然后使用on条件筛选

注：以上结论只在mysql 5.7验证过，其他数据库不一定成立