数据库多表查询

在对数据库进行多表查询时，我们需要将多个数据表联结起来，多表的联结主要有以下几种类型：
1.左联结（left join）：联结结果保留左表中的全部数据
2.右联结（right join）：联结结果保留右表中的全部数据
3.内联结（inner join）：取两个表中的共同数据

使用好以上三种联结类型，我们将能够写出许多精彩复杂的查询操作。

以下有几种情况（阴影部分为我们需要查询的内容），其图示及sql语句写法如下：

select ... from table1 as a
inner join table2 as b
on a.属性列名 = b.属性列名;

select ... from table1 as a
left join table2 as b
on a.属性列名 = b.属性列;

select ... from table1 as a
left join table2 as b
on a.属性列名 = b.属性列名
where b.属性列名 = null;

select ... from table1 as a
right join table2 as b
on a.属性列名 = b.属性列名;

select ... from table1 as a
right join table2 as b
on a.属性列名 = b.属性列名
where a.属性列名 = null;

select ... from table1 as a
full outer join table2 as b
on a.属性列名 = b.属性列名;

select ... from table1 as a
full outer join table2 as b
on a.属性列名 = b.属性列名
where a.属性列名=null or b.属性列名=null;