查询所有学生语文科目成绩,按成绩高低排名
SELECT A.ID,A.NAMES,B.SCORE FROM STUDENT A JOIN SCORES B ON A.ID=B.STUDENT_ID WHERE B.SUBJECT_ID='YW' ORDER BY SCORE DESC
UNION ALL 查询
SELECT * FROM SCORES
UNION ALL //并起来,行数与列数必须相同
SELECT * FROM SCORES2
SELECT STUDENT_ID,SUBJECT_ID,SCORE FROM SCORES
UNION ALL
SELECT STUDENT_ID,SUBJECT_ID,SCORE FROM SCORES2
错误查询，列不相同，如：
SELECT * FROM GRADE
UNION ALL
SELECT * FROM SUBJECT
必须保证列数相同，且数据类型一样
SELECT ID FROM GRADE
UNION ALL
SELECT ID FROM SUBJECT
查询2班所有学生数学成绩>60分的记录
SELECT A.ID,A.NAMES,B.SCORE FROM STUDENT A JOIN SCORES B ON A.ID=B.STUDENT_ID WHERE B.SUBJECT_ID='SX' AND A.GRADE_ID='02' AND B.SCORE>60
查询所有学生信息，显示学生姓名，性别,年龄，班级名称，联系方式，住址
SELECT A.NAMES,CASE A.SEX WHEN 1 THEN '男' WHEN 2 THEN '女' ELSE '录入错误' END AS SEX,YEAR(CURRENT //当前年份DATE)-YEAR(DATE(A.BIRTHDAY)//转换成日期类型) AS AGE,B.NAMES,A.MOBILE,A.ADDRESS FROM STUDENT A JOIN GRADE B ON A.GRADE_ID=B.ID
查询所有学生成绩，按照科目排序，没有成绩的学生分数为0
SELECT C.ID,C.NAMES,C.SUBJECTID,COALESCE(D.SCORE,0) FROM (SELECT A.ID,A.NAMES,B.ID AS SUBJECTID FROM STUDENT A,SUBJECT B) C LEFT JOIN SCORES D ON C.ID = D.STUDENT_ID AND C.SUBJECTID= D.SUBJECT_ID
ORDER BY C.SUBJECTID
//coalesce(c.score,备用的字段)判断为备用的