#!/usr/bin/env python# -*- coding:utf-8 -*-# @time: 2017/11/23 23:10# Author: caicai# @File: demon9.pyimport codecsimport MySQLdbdef 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 cnxif __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.TID2.    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 codecsimport MySQLdbdef 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 cnxif __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：3210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 codecsimport MySQLdbdef 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 cnxif __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。