• day24-python操作数据库四


    #!/usr/bin/env python
    # -*- coding:utf-8 -*-
    # @time: 2017/11/23 23:10
    # Author: caicai
    # @File: demon9.py
    
    import codecs
    
    import MySQLdb
    def connect_mysql():
        db_config = {
            'host': '192.168.1.5',
            'port': 3306,
            'user': 'wxp',
            'passwd': '1qazXSW@',
            'db': 'python',
            'charset': 'utf8'
        }
        cnx = MySQLdb.connect(**db_config)
        return cnx
    
    if __name__ == '__main__':
        cnx = connect_mysql()
    
        sql = '''delete from Teacher where TID in(
        select TID from (select Course.CouID, Course.TID, Teacher.TName, count(Teacher.TID) as count_teacher from Course
        left join Score on Score.Grade < 60 and Course.CouID = Score.CouID
        left join Teacher on Course.TID = Teacher.TID
        group by Course.TID
        order by count_teacher desc
        limit 5)  as test );
        '''
        try:
            cus = cnx.cursor()
            cus.execute(sql)
            result = cus.fetchall()
            cus.close()
            cnx.commit()
        except Exception as e:
            cnx.rollback()
            print('error')
            raise e
        finally:
            cnx.close()
    
    
    结果:
    程序正常执行,没有报错
    解释:
    1.    先查询出Course表中的Course.TID和Course.TID
    2.    left join 是关联Score表,查出Score.Grade > 59,并且,课程ID和课程表的CouID要对应上
    3.    left join Teacher 是关联老师表,课程中的了老师ID和老师表中的老师ID对应上
    4.    select中加上老师的名字Teacher.Tname和count(Teacher.TID)
    5.    group by Course.TID,在根据老师的的TID进行分组
    6.    oder by 最后对count_teacher进行排序,取前5行,
    7.    在通过套用一个select子查询,把所有的TID搂出来
    8.    然后delete from Teacher 最后删除TID在上表中的子查询中。
    import codecs
    
    import MySQLdb
    def connect_mysql():
        db_config = {
            'host': '192.168.1.5',
            'port': 3306,
            'user': 'wxp',
            'passwd': '1qazXSW@',
            'db': 'python',
            'charset': 'utf8'
        }
        cnx = MySQLdb.connect(**db_config)
        return cnx
    
    if __name__ == '__main__':
        cnx = connect_mysql()
    
        sql = '''select *, (grade+60) as newGrade from Score where Grade <5;'''
        update = '''update Score set grade = grade + 60 where grade < 5;  '''
        try:
            cus_start = cnx.cursor()
            cus_start.execute(sql)
            result1 = cus_start.fetchall()
            print(len(result1))
            cus_start.close()
    
            cus_update = cnx.cursor()
            cus_update.execute(update)
            cus_update.close()
    
            cus_end = cnx.cursor()
            cus_end.execute(sql)
            result2 = cus_end.fetchall()
            print(len(result2))
            cus_end.close()
    
            cnx.commit()
        except Exception as e:
            cnx.rollback()
            print('error')
            raise e
        finally:
            cnx.close()
    结果:
    321
    0
    解释:
    1.    刚开始,我们可以查到分数小于5分的总个数有321个
    2.    select *, (grade+60) as newGrade from Score where Grade <5;这个sql是把所有的成绩小于5的都列出来,然后最后加一列分数加60分的结果。
    3.    update Score set grade = grade + 60 where grade < 5;是把分数小于5的所有成绩都加60分
    4.    最后在检查分数小于5的个数为0,说明所有低于5分的分数都发生了改变。
    索引
    索引可以提高查询的速度。
    创建Course的CouID的字段为主键   Score的SID字段为主键    Student的StdID字段为主键  Teacher的TID字段为主键,
    import codecs
    
    import MySQLdb
    def connect_mysql():
        db_config = {
            'host': '192.168.1.5',
            'port': 3306,
            'user': 'wxp',
            'passwd': '1qazXSW@',
            'db': 'python',
            'charset': 'utf8'
        }
        cnx = MySQLdb.connect(**db_config)
        return cnx
    
    if __name__ == '__main__':
        cnx = connect_mysql()
    
        sql1 = '''alter table Teacher add primary key(TID);'''
        sql2 = '''alter table Student add primary key(StdID);'''
        sql3 = '''alter table Score add primary key(SID);'''
        sql4 = '''alter table Course add primary key(CouID);'''
        sql5 = '''alter table Score add index idx_StdID_CouID(StdID, CouID);'''
        # sql6 = '''alter table Score drop  index idx_StdID_CouID;'''   删除索引
        sql7 = '''explain select * from Score where StdID = 16213;'''
        try:
            cus = cnx.cursor()
            cus.execute(sql1)
            cus.close()
    
            cus = cnx.cursor()
            cus.execute(sql2)
            cus.close()
    
            cus = cnx.cursor()
            cus.execute(sql3)
            cus.close()
    
            cus = cnx.cursor()
            cus.execute(sql4)
            cus.close()
    
            cus = cnx.cursor()
            cus.execute(sql5)
            cus.close()
    
            cus = cnx.cursor()
            cus.execute(sql7)
            result = cus.fetchall()
            print(result)
            cus.close()
    
            cnx.commit()
        except Exception as e:
            cnx.rollback()
            print('error')
            raise e
        finally:
            cnx.close()
    
    结果:
    ((1L, u'SIMPLE', u'Score', u'ref', u'idx_StdID_CouID', u'idx_StdID_CouID', u'4', u'const', 4L, None),)
    解释:
    Sql1, sql2, sql3, sql4是添加主键,sql5是增加一个索引,我们也可以在mysql的客户端上执行sq7,得到如下的结果:
    mysql> explain select * from Score where StdID = 16213;               
    +----+-------------+-------+------+-----------------+-----------------+---------+-------+------+-------+
    | id | select_type | table | type | possible_keys   | key             | key_len | ref   | rows | Extra |
    +----+-------------+-------+------+-----------------+-----------------+---------+-------+------+-------+
    |  1 | SIMPLE      | Score | ref  | idx_StdID_CouID | idx_StdID_CouID | 4       | const |    4 | NULL  |
    +----+-------------+-------+------+-----------------+-----------------+---------+-------+------+-------+
    1 row in set (0.00 sec)
    这个说明,我们在搜索StdID的时候,是走了idx_StdID_CouID索引的。
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  • 原文地址:https://www.cnblogs.com/wxp997/p/7887666.html
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