笛卡爾連接或交叉連接從兩個(gè)或多個(gè)連接表返回笛卡爾乘積的記錄。因此,它相當(dāng)于一個(gè)內(nèi)部聯(lián)接,其中聯(lián)接條件始終計(jì)算為真或者聯(lián)接條件是從語句中空缺。
笛卡爾連接的基本語法如下:
SELECT table1.column1, table2.column2... FROM table1, table2 [, table3 ]
考慮下面的兩個(gè)表中,(a)CUSTOMERS表如下:
+----+----------+-----+-----------+----------+ | ID | NAME | AGE | ADDRESS | SALARY | +----+----------+-----+-----------+----------+ | 1 | Ramesh | 32 | Ahmedabad | 2000.00 | | 2 | Khilan | 25 | Delhi | 1500.00 | | 3 | kaushik | 23 | Kota | 2000.00 | | 4 | Chaitali | 25 | Mumbai | 6500.00 | | 5 | Hardik | 27 | Bhopal | 8500.00 | | 6 | Komal | 22 | MP | 4500.00 | | 7 | Muffy | 24 | Indore | 10000.00 | +----+----------+-----+-----------+----------+
(b)另一個(gè)ORDERS 表如下:
+-----+---------------------+-------------+--------+ |OID | DATE | CUSTOMER_ID | AMOUNT | +-----+---------------------+-------------+--------+ | 102 | 2009-10-08 00:00:00 | 3 | 3000 | | 100 | 2009-10-08 00:00:00 | 3 | 1500 | | 101 | 2009-11-20 00:00:00 | 2 | 1560 | | 103 | 2008-05-20 00:00:00 | 4 | 2060 | +-----+---------------------+-------------+--------+
現(xiàn)在,讓我們使用笛卡爾連接在這兩個(gè)表如下:
SQL> SELECT ID, NAME, AMOUNT, DATE FROM CUSTOMERS, ORDERS;
這將產(chǎn)生以下結(jié)果:
+----+----------+--------+---------------------+ | ID | NAME | AMOUNT | DATE | +----+----------+--------+---------------------+ | 1 | Ramesh | 3000 | 2009-10-08 00:00:00 | | 1 | Ramesh | 1500 | 2009-10-08 00:00:00 | | 1 | Ramesh |