oracle数据库sql中的where条件执行顺序

本文链接： https://blog.csdn.net/Kesa_Wu/article/details/102671760

采用自下而上的顺序解析where子句

|
|--(根据这个原理)
|--1.表之间的连接必须写在其他where条件之前
|--2.那些可以过滤掉最大数量记录的条件必须写在where子句的末尾

引发效率问题举例

|
|--(低效)|---select … from emp e where 
|        |   	   sal > 50000     
|        |   and job = 'manager'
|        |   and 25 < (select count(*) from emp where mgr=e.empno); 　
|
|--(高效)|---select … from emp e where         
|        |       25 < (select count(*) from emp where mgr=e.empno)
|        |   and sal > 50000     
|        |   and job = 'manager'; 

引发正确性问题举例

|
|\
|  \______table:demo_________________
|  |  id   id01      id02     id03   |
|  |  1     55       55       md     |
|  |  2     66       abced    eeeee  |
|  |__3_____300______200______rt_____|
|
|--报错语句|---update demo a set id01 = 100 where
|          |   	a.id03 in ('md','rt')     
|          |   and to_number(a.id02) < 100;
|          --------------------------------------------------(to_number(a.id02),转换异常)
| 
|--正确语句|---update demo a set id01 = 100 where
|          |   	to_number(a.id02) < 100     
|          |   and a.id03 in ('md','rt');