PostgreSQL中的整除截断

开发同学说，在自己的测试环境中，相同的数据和相同的sql，在mysql和pg中查询出来的结果却不同。问是不是他装的数据库有问题了或者sum函数用错了。

以下是测试数据：

postgres=# select * from emp;
 empno | ename  |    job    | mgr  |  hiredate  | sal  | comm | deptno 
-------+--------+-----------+------+------------+------+------+--------
  7369 | SMITH  | CLERK     | 7902 | 1980-12-17 |  800 |      |     20
  7499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 | 1600 |  300 |     30
  7521 | WARD   | SALESMAN  | 7698 | 1981-02-22 | 1250 |  500 |     30
  7566 | JONES  | MANAGER   | 7839 | 1981-04-02 | 2975 |      |     20
  7654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 | 1250 | 1400 |     30
  7698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 | 2850 |      |     30
  7782 | CLARK  | MANAGER   | 7839 | 1981-06-09 | 2450 |      |     10
  7788 | SCOTT  | ANALYST   | 7566 | 1982-12-09 | 3000 |      |     20
  7839 | KING   | PRESIDENT |      | 1981-11-17 | 5000 |      |     10
  7844 | TURNER | SALESMAN  | 7698 | 1981-09-08 | 1500 |    0 |     30
  7876 | ADAMS  | CLERK     | 7788 | 1983-01-12 | 1100 |      |     20
  7900 | JAMES  | CLERK     | 7698 | 1981-12-03 |  950 |      |     30
  7902 | FORD   | ANALYST   | 7566 | 1981-12-03 | 3000 |      |     20
  7934 | MILLER | CLERK     | 7782 | 1982-01-23 | 1300 |      |     10
(14 rows)

postgres=# select (sum(case when deptno=10 then sal end)/sum(sal))*100 as pct from emp;
 pct 
-----
   0
(1 row)

postgres=#

和

mysql> select * from emp;
+-------+--------+-----------+------+------------+------+------+--------+
| EMPNO | ENAME  | JOB       | MGR  | HIREDATE   | SAL  | COMM | DEPTNO |
+-------+--------+-----------+------+------------+------+------+--------+
|  7369 | SMITH  | CLERK     | 7902 | 1980-12-17 |  800 | NULL |     20 |
|  7499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 | 1600 |  300 |     30 |
|  7521 | WARD   | SALESMAN  | 7698 | 1981-02-22 | 1250 |  500 |     30 |
|  7566 | JONES  | MANAGER   | 7839 | 1981-04-02 | 2975 | NULL |     20 |
|  7654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 | 1250 | 1400 |     30 |
|  7698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 | 2850 | NULL |     30 |
|  7782 | CLARK  | MANAGER   | 7839 | 1981-06-09 | 2450 | NULL |     10 |
|  7788 | SCOTT  | ANALYST   | 7566 | 1982-12-09 | 3000 | NULL |     20 |
|  7839 | KING   | PRESIDENT | NULL | 1981-11-17 | 5000 | NULL |     10 |
|  7844 | TURNER | SALESMAN  | 7698 | 1981-09-08 | 1500 |    0 |     30 |
|  7876 | ADAMS  | CLERK     | 7788 | 1983-01-12 | 1100 | NULL |     20 |
|  7900 | JAMES  | CLERK     | 7698 | 1981-12-03 |  950 | NULL |     30 |
|  7902 | FORD   | ANALYST   | 7566 | 1981-12-03 | 3000 | NULL |     20 |
|  7934 | MILLER | CLERK     | 7782 | 1982-01-23 | 1300 | NULL |     10 |
+-------+--------+-----------+------+------------+------+------+--------+
14 rows in set (0.00 sec)

mysql> select (sum(case when deptno=10 then sal end)/sum(sal))*100 as pct from emp;
+---------+
| pct     |
+---------+
| 30.1464 |
+---------+
1 row in set (0.00 sec)

mysql>

其实，这个和数值整除有关。

先看看mysql中对“/”的定义：

再看看pg中的定义：

此外，这里虽然不是sum()的问题，但是mysql和pg中，sum()的返回值也的确有所区别：

先看看mysql中对“sum”的定义：

再看看pg中的定义：