1) . select * from demo where id<=10 and name='zhangsan' and (sex='0' or 'age'>=18);
2).select * from demo where id<=10 and name='zhangsan' and sex='0' or 'age'>=18;
3).select * from demo where id<=10 and name='zhangsan' and sex='0' and 'age'>=18;
4). select * from demo where id<=10 and name='zhangsan' and (sex='0' and 'age'>=18);
我弄不清楚 1和2的区别 3和4的区别，谢谢大神，能指点下吗？
括号的优先级高
http://dev.mysql.com/doc/refman/5.1/zh/functions.html#operator-precedence
如楼上所说的，由于括号的问题，执行where字句的顺序也不同了，可能会影响效率