mysql组合索引的命中情况

在account表中拿取3个字段  A,B ,C建立组合索引 

测试sql : desc select id from account where a=1 and b=2 and c=3 ; 

desc select id from account where a=1 and b=2 ; 

desc select id from account where a=1  ; 

这三种情况都是命中的 

 

desc select id from account where b=2 and c=3; 

desc select id from account where c=3;

这两种情况都是不命中的.