sql语句优化

1. sql语句与orcale语句的where语句的条件执行顺序 : 都是从右到左执行

示例:

1. mysql:

SELECT * FROM `book` where category is NULL or category='文学' and price>80;     结果是0条

SELECT * FROM `book` where price>80  and category is NULL or category='文学' ; 结果是2条

2.oracle

Select 'ok' From Dual Where 1 / 0 = 1 And 1 = 2;  执行

Select 'ok' From Dual Where 1 = 2 And 1 / 0 = 1;  出错(除数为0)