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mysql where子句

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where 是针对元素进行操作，所有where之后的语句，都是针对数据结果进行操作，而不是是针对数据表

	--where子句 

	---查询学生id为1,3,5,6,

	select * from student where id = 1 or id = 3 or id = 5 or id = 6;

	----简化

	select * from student where id in (1,3,5,6);

	--between and 

	between A and B(A 必须小于等于B) 

	--查询2-5id的学生

	select * from student where id 2 and 5;

	--null运算

	---select 1+null;

	---select 2*null;

	---select 3 and null

	---结果

	--查询id为null的学生

	--错误 

	select * from student where class_id = null;

	--正确

	select * from student where class_id is null;