#!/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'', 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。