every derived table must have its own alias

好比现有一张表user:

id    name    age

1       a           10

2       b           15  

3       c           20

4       a           10

5       b           15

6       a           15

7       b           10

要求按name和age都相同的去重后统计条目，如上表去重后总行数应该为5。

最初设计的sql语句为select count(name) from (select name,age,count(name) from user group by name,age);

结果运行时报错：every derived table must have its own alias。意思是每个派生出的表格都要有一个别名，因而咱们给去重后的表格起个别名subtable，修改后的sql语句以下：

select count(name) from (select name,age,count(name) from user group by name,age)as subtable;