聚集的语法规则各数据库有差异。

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：

snippet.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：

snippet.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，

snippet.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。