//SQL语句
//1. select 学号, 分数 from grade
// where 90>=分数 and 分数 >=80
//2. select AVG(分数) from grade where 课程编号 ='0002'
//3. select COUNT(*) from grade where 课程编号 ='0001'
// select COUNT(*) from grade where 课程编号 ='0002'
//4. select 学号,姓名 from student_info
// where 姓名 like '张%'
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