笔试题-sql语句

Posted 2020-10-26 kangkaii

今天遇到了不熟练（不会）的查询题目

• 回来自己又做了一下，如下

建表语句

---

-- Table structure for score

---

DROP TABLE IF EXISTS score;
CREATE TABLE score (
id int(11) DEFAULT NULL,
sno int(11) DEFAULT NULL,
name varchar(255) DEFAULT NULL,
subject varchar(255) DEFAULT NULL,
score varchar(255) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

---

-- Records of score

---

INSERT INTO score VALUES (\'1\', \'201201\', \'小明\', \'math\', \'90\');
INSERT INTO score VALUES (\'2\', \'201202\', \'小李\', \'math\', \'99\');
INSERT INTO score VALUES (\'3\', \'201203\', \'小红\', \'math\', \'43\');
INSERT INTO score VALUES (\'4\', \'201201\', \'小明\', \'en\', \'66\');
INSERT INTO score VALUES (\'5\', \'201202\', \'小李\', \'en\', \'56\');
INSERT INTO score VALUES (\'6\', \'201203\', \'小红\', \'en\', \'88\');
INSERT INTO score VALUES (\'7\', \'201201\', \'小明\', \'computer\', \'99\');
INSERT INTO score VALUES (\'8\', \'201202\', \'小李\', \'computer\', \'65\');
INSERT INTO score VALUES (\'9\', \'201203\', \'小红\', \'computer\', \'67\');
INSERT INTO score VALUES (\'11\', \'201204\', \'小白\', \'sss\', \'99\');
INSERT INTO score VALUES (\'11111\', \'201203\', \'小红\', \'computer\', \'67\');

• 题目1：找到所有分数都及格（>=60）的学生。

  1. 查询有科目分数小于 60 的， 然后 not in 即可
     select name from score where sno not in
     (select s.sno from score s where s.score < 60) GROUP BY sno

  2. not exists 写法
     select name from score s1 where not exists
     (select 1 from score s where s.score < 60 and s1.sno = s.sno) GROUP BY s1.sno

  3. having 筛选
     select * from score s GROUP BY s.sno having min(s.score) >= 60

• 题目2：表中有冗余记录，如何删除？如下图：

分析：找到该数据对应的id，根据id删除！

select s.id from score s group by s.sno,s.name,s.subject,s.score -- 此结果不包含需要删除的id
本以为是：delete from score where id not in (select s.id from score s group by s.sno,s.name,s.subject,s.score )

• 结果毫不犹豫的抛出错误：
  

• 查明原因，mysql认为delete/update 跟 子句里面select到的不能是一张表

• 做出改进（表起别名，按照需要删掉新插入的或者后插入的）

delete from score where id not in (select id from (select min(id) id from score s group by sno,name,subject,score )t)