Suppose the order business table structure is:
order(oid, date, uid, status, money, time, …)
in:
oid , order id , primary key
date , the date of the order, there is a common index , the management background often queries according to date
uid , user ID , there is a common index , users can query their own orders
status , the order status, there is a common index , the management background is often queried according to the status
money/time , order amount /time , queried field, no index
…
Suppose an order has three states: 0 Ordered, 1 Paid, 2 Completed
Business requirements, query outstanding orders, which SQL is faster?
select * from order where status!=2
select * from order where status=0 or status=1
select * from order where status IN (0,1)
select * from order where status=0
union all
select * from order where status=1
Conclusion: Scheme 1 is the slowest, schemes 2, 3, and 4 can all hit the index
but...
One: union all must be able to hit the index
select * from order where status=0
union all
select * from order where status=1
illustrate:
Tell MySQL directly what to do, MySQL consumes the least CPU
Programmers don't often write SQL like this (union all)
Two: simple in can hit the index
select * from order where status in (0,1)
illustrate:
Let MySQL think, query optimization consumes more CPU than union all , but it can be ignored
Programmers often write SQL(in) like this. In this example, it is most recommended to write like this
Three: For or , the new version of MySQL can hit the index
select * from order where status=0 or status=1
illustrate:
让MySQL思考，查询优化耗费的cpu比in多，别把负担交给MySQL
不建议程序员频繁用or，不是所有的or都命中索引
对于老版本的MySQL，建议查询分析下
四、对于!=，负向查询肯定不能命中索引
select * from order where status!=2
说明：
全表扫描，效率最低，所有方案中最慢
禁止使用负向查询
五、其他方案
select * from order where status < 2
这个具体的例子中，确实快，但是：
这个例子只举了3个状态，实际业务不止这3个状态，并且状态的“值”正好满足偏序关系，万一是查其他状态呢，SQL不宜依赖于枚举的值，方案不通用
这个SQL可读性差，可理解性差，可维护性差，强烈不推荐