select * from a,b探討

select * from a,b探討

今天看同事代碼里使用了select * from a,b where a.id=b.id，而我平時都是使用select * from a inner join b where a.id=b.id，於是查了下，發現：

1）單純的select * from a,b是笛卡爾乘積

2）select * from a,b where a.id=b.id相當於inner join

#### 驗證

1）創建兩張表

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）插入測試數據

insert into userinfo values(1,1),(2,1),(3,2),(4,6);
insert into report values(1,'第一條'),(2,'第二條'),(3,'第三條');

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 | 第一條      |
|         2 | 第二條      |
|         3 | 第三條      |
+-----------+-------------+
3 rows in set (0.00 sec)

3）驗證

單獨的select * from a,b

select * from userinfo,report

結果

mysql> select * from userinfo,report;
+-----+-----------+-----------+-------------+
| uid | report_id | report_id | description |
+-----+-----------+-----------+-------------+
|   1 |         1 |         1 | 第一條      |
|   1 |         1 |         2 | 第二條      |
|   1 |         1 |         3 | 第三條      |
|   2 |         1 |         1 | 第一條      |
|   2 |         1 |         2 | 第二條      |
|   2 |         1 |         3 | 第三條      |
|   3 |         2 |         1 | 第一條      |
|   3 |         2 |         2 | 第二條      |
|   3 |         2 |         3 | 第三條      |
|   4 |         6 |         1 | 第一條      |
|   4 |         6 |         2 | 第二條      |
|   4 |         6 |         3 | 第三條      |
+-----+-----------+-----------+-------------+
12 rows in set (0.00 sec)

可見select * from a,b是笛卡兒積

再來驗證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 | 第一條      |
|   2 |         1 |         1 | 第一條      |
|   3 |         2 |         2 | 第二條      |
+-----+-----------+-----------+-------------+
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 | 第一條      |
|   2 |         1 |         1 | 第一條      |
|   3 |         2 |         2 | 第二條      |
+-----+-----------+-----------+-------------+
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 | 第一條      |
|   2 |         1 |         1 | 第一條      |
|   3 |         2 |         2 | 第二條      |
+-----+-----------+-----------+-------------+
3 rows in set (0.00 sec)

可見是select * from a,b where a.id=b.id只是把笛卡爾積做了一層過濾，結果與inner join相同

補充：inner join是先生成一個臨時表，然后使用on條件篩選

注：以上結論只在mysql 5.7驗證過，其他數據庫不一定成立