1、单表和多表查询

        1.1 算术运算符、比较运算符及特殊运算符

          1)MySQL的算术运算符

          

select 0.1+0.3333,0.1-0.3333,0.1*0.3333,1/2,1%2;

select 1/0,100%0;

select 3%2,mod(3,2);

        2）MySQL的比较运算符

select 1=0,1=1,null=null;

select 1<>0,1<>1,null<>null;

select 1<=>1,2<=>0,0<=>0,null<=>null;

select ‘a’<‘b’,’a’<‘a’,’a’<‘c’,1<2;

select ‘bdf’>’b’,’b’,<=‘b’,0<1;

select ‘a’>’b’,’abc’&g