查询树的启发式优化（代数优化算法）

示例一

SELECT Student.Sname
FROM Student，SC 
WHERE Student.Sno=SC.Sno AND SC.Cno='2'；

示例二

SELECT Sname
	FROM Student, SC, Course
	WHERE Student.Sno=SC.Sno AND
		SC.Cno=Course.Cno AND 
		course.Cname=‘C语言’;

示例三

SELECT Student.Sname
	FROM Student，SC， Course
	WHERE Student.Sno=SC.Sno and Course.Cno= SC.Cno
			and Student.Dept='计算机学院' and Course.Cname='DataBase'