条件查询:
	7.1 逻辑条件: and(与)  or(或)
	SELECT * FROM student WHERE id=2 AND NAME='李四';
	SELECT * FROM student WHERE id=2 OR NAME='张三';

	7.2 比较条件: >  <  >=  <=  ==  <>(不等于)  between and
	SELECT * FROM student WHERE jsp>75 AND jsp<=90;
	SELECT * FROM student WHERE jsp BETWEEN 75 AND 90;

	7.3 判空条件(null 空字符串): is null  /  is not null  /  =''  /  <>''
	SELECT * FROM student WHERE address IS NULL OR address='';

	7.4 模糊查询: like
	%:表示任意个字符
	_:表示一个字符
	SELECT * FROM student WHERE name LIKE '李%';