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。