运算符的优先级

很久没有去深究运算符的优先级了，今天写SQL解析思考了一下。

下面这条SQL真正的执行顺序是什么？

select * from taobao_category where type=1 or id=801 and type=3 and type=4

 

改成python脚本：

def test(type,id):
    print type==1 or id==801 and id==802 and id==803

 测试结果是True

 

基本所有的语言的优先级都是 not>and>or，

所以上面那段python表达式实际是:  (type==1) or (id==801 and id==802 and id==803)，sql也就是：

select * from taobao_category where (type=1) or (id=801 and type=3 and type=4)

mysql> select * from taobao_category where type=0 and type=1 or id=324 and id =23 or id=830;
+-----+----------+------+-----------+-----------------+------+------+--------+
| id  | cid      | type | parent_id | name            | url  | des  | status |
+-----+----------+------+-----------+-----------------+------+------+--------+
| 830 | 50024129 |    0 |      1402 | - | NULL | NULL |   NULL |
+-----+----------+------+-----------+-----------------+------+------+--------+
1 row in set (0.00 sec)