mysql学习（十）多表查询

多表查询时，要给表名起别名,给字段名其别名（当两个表含有重复字段时）

select p.name, c.name, pid from products p, cats c;//得到的结果为笛卡尔乘积

1. select p.name as pname, p.price, p.desn, p.num from products p, cats c where c.id = p.cid ;//查询所有商品属于哪个分类
2. select p.name as pname, p.price, p.desn, p.num from products p, cats c where c.id = p.cid and c.name like '%java%'//查询类名中含有java的商品信息

3. mysql> select * from cats;
   +----+-----+-------+-----------+
   | id | pid | name | desn |
   +----+-----+-------+-----------+
   | 4 | 0 | soft | this soft |
   | 5 | 4 | java | this java |
   | 6 | 4 | php | this java |
   | 7 | 4 | c++ | this java |
   | 8 | 5 | j2ee | j2ee |
   | 9 | 5 | j2me | j2me |
   | 10 | 9 | j2me1 | j2me1 |
   +----+-----+-------+-----------+

4. mysql> select a.id aid, a.name aname, b.id bid, b.name bname from cats a, cats b where a.pid = b.id;//查询所有分类
   +-----+-------+-----+-------+
   | aid | aname | bid | bname |
   +-----+-------+-----+-------+
   | 7 | c++ | 4 | soft |
   | 8 | j2ee | 5 | java |
   | 9 | j2me | 5 | java |
   | 10 | j2me1 | 9 | j2me |
   | 5 | java | 4 | soft |
   | 6 | php | 4 | soft |
   +-----+-------+-----+-------+
   6 rows in set (0.00 sec)