# 1. 聚集语法规则

聚集的语法规则各数据库有差异。

# 2. 投影列规则
在有Group By的情况下，投影列要么出现在Group By中，要么在聚集函数中，例如，以下SQL是合法的：

```sql
select sum(a) from t group by a;
select sum(a),a from t group by a;
select sum(b),a from t group by a;
select sum(a+b) from t group by a+b;
select sum(a+b),a+b from t group by a+b;
select a+b from t group by a+b;
select a,b from t group by a,b;
```

以下SQL是非合法的：

```sql
-- 投影列b既不在聚集函数中，也不在Group By中：
select sum(a),b from t group by a;
select * from t group by a;
select b from t group by a;
select a,b from t group by a;
select sum(a+b),a from t group by a+b;
```

有些数据库，支持上述非法SQL，例如，

```sql
select b from t group by a; 
```


因为a的每个分组中可能对应多个b值，返回结果b列显示第一个值。