板凳-------Mysql cookbook学习 （八）
MySQL ：： MySQL 8.0 参考手册 ：： 30.4.5.1 extract_schema_from_file_name（） 函数
第3章：从表中查询数据
3.0 引言
mysql> use cookbook
Database changed
mysql> select version(), database();
+-----------+------------+
| version() | database() |
+-----------+------------+
| 8.0.40 | cookbook |
+-----------+------------+
1 row in set (0.00 sec)mysql> select * from mail;
+---------------------+---------+---------+---------+---------+---------+
| t | srcuser | srchost | dstuser | dsthost | size |
+---------------------+---------+---------+---------+---------+---------+
| 2006-05-11 10:15:08 | barb | saturn | tricia | mars | 58274 |
| 2006-05-12 12:48:13 | tricia | mars | gene | venus | 194925 |
| 2006-05-12 15:02:49 | phil | mars | phil | saturn | 1048 |
| 2006-05-13 13:59:18 | barb | saturn | tricia | venus | 271 |
| 2006-05-14 09:31:37 | gene | venus | barb | mars | 2291 |
| 2006-05-14 11:52:17 | phil | mars | tricia | saturn | 5781 |
| 2006-05-14 14:42:21 | barb | venus | barb | venus | 98151 |
| 2006-05-14 17:03:01 | tricia | saturn | phil | venus | 2394482 |
| 2006-05-15 07:17:48 | gene | mars | gene | saturn | 3824 |
| 2006-05-15 08:50:57 | phil | venus | phil | venus | 978 |
| 2006-05-15 10:25:52 | gene | mars | tricia | saturn | 998532 |
| 2006-05-15 17:35:31 | gene | saturn | gene | mars | 3856 |
| 2006-05-16 09:00:28 | gene | venus | barb | mars | 613 |
| 2006-05-16 23:04:19 | phil | venus | barb | venus | 10294 |
| 2006-05-17 12:49:23 | phil | mars | tricia | saturn | 873 |
| 2006-05-19 22:21:51 | gene | saturn | gene | venus | 23992 |
+---------------------+---------+---------+---------+---------+---------+
16 rows in set (0.04 sec)
3.1 指定查询列/从指定列中查询
显示查询列， 可以按用户指定的任意顺序显示
mysql> select t, srchost, srcuser, dsthost, dstuser, size from mail;
+---------------------+---------+---------+---------+---------+---------+
| t | srchost | srcuser | dsthost | dstuser | size |
+---------------------+---------+---------+---------+---------+---------+
| 2006-05-11 10:15:08 | saturn | barb | mars | tricia | 58274 |
| 2006-05-12 12:48:13 | mars | tricia | venus | gene | 194925 |
| 2006-05-12 15:02:49 | mars | phil | saturn | phil | 1048 |
| 2006-05-13 13:59:18 | saturn | barb | venus | tricia | 271 |
| 2006-05-14 09:31:37 | venus | gene | mars | barb | 2291 |
| 2006-05-14 11:52:17 | mars | phil | saturn | tricia | 5781 |
| 2006-05-14 14:42:21 | venus | barb | venus | barb | 98151 |
| 2006-05-14 17:03:01 | saturn | tricia | venus | phil | 2394482 |
| 2006-05-15 07:17:48 | mars | gene | saturn | gene | 3824 |
| 2006-05-15 08:50:57 | venus | phil | venus | phil | 978 |
| 2006-05-15 10:25:52 | mars | gene | saturn | tricia | 998532 |
| 2006-05-15 17:35:31 | saturn | gene | mars | gene | 3856 |
| 2006-05-16 09:00:28 | venus | gene | mars | barb | 613 |
| 2006-05-16 23:04:19 | venus | phil | venus | barb | 10294 |
| 2006-05-17 12:49:23 | mars | phil | saturn | tricia | 873 |
| 2006-05-19 22:21:51 | saturn | gene | venus | gene | 23992 |
+---------------------+---------+---------+---------+---------+---------+
16 rows in set (0.00 sec)
3.2 指定查询行
为了更精确的查询， 通过使用where 语句， 给select 查询加上一个或多个条件限制。
mysql> select t, srcuser, srchost, size from mail where srchost = 'venus';
+---------------------+---------+---------+-------+
| t | srcuser | srchost | size |
+---------------------+---------+---------+-------+
| 2006-05-14 09:31:37 | gene | venus | 2291 |
| 2006-05-14 14:42:21 | barb | venus | 98151 |
| 2006-05-15 08:50:57 | phil | venus | 978 |
| 2006-05-16 09:00:28 | gene | venus | 613 |
| 2006-05-16 23:04:19 | phil | venus | 10294 |
+---------------------+---------+---------+-------+
5 rows in set (0.00 sec)mysql> select t, srcuser, srchost from mail where srchost like 's%';
+---------------------+---------+---------+
| t | srcuser | srchost |
+---------------------+---------+---------+
| 2006-05-11 10:15:08 | barb | saturn |
| 2006-05-13 13:59:18 | barb | saturn |
| 2006-05-14 17:03:01 | tricia | saturn |
| 2006-05-15 17:35:31 | gene | saturn |
| 2006-05-19 22:21:51 | gene | saturn |
+---------------------+---------+---------+
5 rows in set (0.00 sec)
mysql> select * from mail where srcuser = 'barb' and dstuser = 'tricia';
+---------------------+---------+---------+---------+---------+-------+
| t | srcuser | srchost | dstuser | dsthost | size |
+---------------------+---------+---------+---------+---------+-------+
| 2006-05-11 10:15:08 | barb | saturn | tricia | mars | 58274 |
| 2006-05-13 13:59:18 | barb | saturn | tricia | venus | 271 |
+---------------------+---------+---------+---------+---------+-------+
3.3 格式化显示查询结果
使用列别名来作为所选择的名称
mysql> select t, srcuser, size from mail;
+---------------------+---------+---------+
| t | srcuser | size |
+---------------------+---------+---------+
| 2006-05-11 10:15:08 | barb | 58274 |
| 2006-05-12 12:48:13 | tricia | 194925 |
| 2006-05-12 15:02:49 | phil | 1048 |
| 2006-05-13 13:59:18 | barb | 271 |
| 2006-05-14 09:31:37 | gene | 2291 |
| 2006-05-14 11:52:17 | phil | 5781 |
| 2006-05-14 14:42:21 | barb | 98151 |
| 2006-05-14 17:03:01 | tricia | 2394482 |
| 2006-05-15 07:17:48 | gene | 3824 |
| 2006-05-15 08:50:57 | phil | 978 |
| 2006-05-15 10:25:52 | gene | 998532 |
| 2006-05-15 17:35:31 | gene | 3856 |
| 2006-05-16 09:00:28 | gene | 613 |
| 2006-05-16 23:04:19 | phil | 10294 |
| 2006-05-17 12:49:23 | phil | 873 |
| 2006-05-19 22:21:51 | gene | 23992 |
+---------------------+---------+---------+
16 rows in set (0.00 sec)
使用一个运算表达式在查询结果中插入一列， 表达式作为查询结果的默认列名
这样可能导致查询结果中有的列名长且含义不明确。
mysql> select-> concat (monthname(t), '', dayofmonth(t), ',', year(t)),-> srcuser,size from mail;
+--------------------------------------------------------+---------+---------+
| concat (monthname(t), '', dayofmonth(t), ',', year(t)) | srcuser | size |
+--------------------------------------------------------+---------+---------+
| May11,2006 | barb | 58274 |
| May12,2006 | tricia | 194925 |
| May12,2006 | phil | 1048 |
| May13,2006 | barb | 271 |
| May14,2006 | gene | 2291 |
| May14,2006 | phil | 5781 |
| May14,2006 | barb | 98151 |
| May14,2006 | tricia | 2394482 |
| May15,2006 | gene | 3824 |
| May15,2006 | phil | 978 |
| May15,2006 | gene | 998532 |
| May15,2006 | gene | 3856 |
| May16,2006 | gene | 613 |
| May16,2006 | phil | 10294 |
| May17,2006 | phil | 873 |
| May19,2006 | gene | 23992 |
+--------------------------------------------------------+---------+---------+
16 rows in set (0.01 sec)
使用date_format 可以得到一样的查询结果。
mysql> select-> date_format(t, '%m %e, %y'),-> srcuser, size from mail;
+-----------------------------+---------+---------+
| date_format(t, '%m %e, %y') | srcuser | size |
+-----------------------------+---------+---------+
| 05 11, 06 | barb | 58274 |
| 05 12, 06 | tricia | 194925 |
| 05 12, 06 | phil | 1048 |
| 05 13, 06 | barb | 271 |
| 05 14, 06 | gene | 2291 |
| 05 14, 06 | phil | 5781 |
| 05 14, 06 | barb | 98151 |
| 05 14, 06 | tricia | 2394482 |
| 05 15, 06 | gene | 3824 |
| 05 15, 06 | phil | 978 |
| 05 15, 06 | gene | 998532 |
| 05 15, 06 | gene | 3856 |
| 05 16, 06 | gene | 613 |
| 05 16, 06 | phil | 10294 |
| 05 17, 06 | phil | 873 |
| 05 19, 06 | gene | 23992 |
+-----------------------------+---------+---------+
16 rows in set (0.00 sec)
别名可以是任意的一个单词，也可以是词组，但同时也要遵守一定的定义规则。
mysql> select-> date_format(t, '%m %e, %y') as 'date of message',-> srcuser as 'message sender', size as 'number of bytes' from mail;
+-----------------+----------------+-----------------+
| date of message | message sender | number of bytes |
+-----------------+----------------+-----------------+
| 05 11, 06 | barb | 58274 |
| 05 12, 06 | tricia | 194925 |
| 05 12, 06 | phil | 1048 |
| 05 13, 06 | barb | 271 |
| 05 14, 06 | gene | 2291 |
| 05 14, 06 | phil | 5781 |
| 05 14, 06 | barb | 98151 |
| 05 14, 06 | tricia | 2394482 |
| 05 15, 06 | gene | 3824 |
| 05 15, 06 | phil | 978 |
| 05 15, 06 | gene | 998532 |
| 05 15, 06 | gene | 3856 |
| 05 16, 06 | gene | 613 |
| 05 16, 06 | phil | 10294 |
| 05 17, 06 | phil | 873 |
| 05 19, 06 | gene | 23992 |
+-----------------+----------------+-----------------+
16 rows in set (0.00 sec)
mysql> select '1+1+1' as 'The expression', 1+1+1 as 'The result';
+----------------+------------+
| The expression | The result |
+----------------+------------+
| 1+1+1 | 3 |
+----------------+------------+
1 row in set (0.00 sec)mysql> select 1 as interger;
+----------+
| interger |
+----------+
| 1 |
+----------+
1 row in set (0.00 sec)mysql> select 1 as 'integer';
+---------+
| integer |
+---------+
| 1 |
+---------+
1 row in set (0.00 sec)
3.4 使用列别名来简化程序
通过使用别名的方法， 给所有使用的查询结果列一个简单的列名，方便引用。
主要修正点：
- 统一使用 mysql.connector：删除了不必要的 MySQLdb 导入，统一使用 mysql.connector
- 正确导入模块：添加了 import mysql.connector 和 from mysql.connector import Error
- 修复配置处理：
o 使用单独的 db_config 变量存储配置
o 修正了配置文件的读取方式 - 完善异常处理：
o 初始化 connection = None
o 在 finally 块中添加了 if connection 检查 - 函数调用位置：将函数调用放在最后
import os
import configparser
import mysql.connector
from mysql.connector import Errordef query_mail_data():# --- 配置部分 ---# 方法1：直接硬编码配置（适合测试）config = {'host': 'localhost','user': 'your_username','password': 'your_password','database': 'cookbook'}# 方法2：从配置文件读取（推荐）config_path = 'D:/sql/Mysql_learning/config.ini'if os.path.exists(config_path):config_parser = configparser.ConfigParser()config_parser.read(config_path)db_config = {'host': config_parser.get('database', 'host', fallback='localhost'),'user': config_parser.get('database', 'user'),'password': config_parser.get('database', 'password'),'database': config_parser.get('database', 'database', fallback='cookbook')}else:print(f"警告：配置文件 {config_path} 不存在，使用默认配置")db_config = config # 回退到硬编码配置# --- 数据库连接部分 ---connection = Nonetry:# 建立数据库连接connection = mysql.connector.connect(host=db_config['host'],database=db_config['database'],user=db_config['user'],password=db_config['password'])if connection.is_connected():cursor = connection.cursor(dictionary=True) # 使用字典游标方便按列名访问# 执行查询query = """SELECT DATE_FORMAT(t, '%m %e, %y') AS 'date of message',srcuser AS 'message sender', size AS 'number of bytes' FROM mail"""cursor.execute(query)# 获取所有结果records = cursor.fetchall()# 打印结果print("\n邮件信息:")print("-" * 50)for row in records:print(f"日期: {row['date of message']}")print(f"发件人: {row['message sender']}")print(f"字节数: {row['number of bytes']}")print("-" * 50)except Error as e:print(f"数据库错误: {e}")finally:if connection and connection.is_connected():cursor.close()connection.close()print("MySQL连接已关闭")# 运行查询
query_mail_data()
邮件信息:
--------------------------------------------------
日期: 05 11, 06
发件人: barb
字节数: 58274
--------------------------------------------------
日期: 05 12, 06
发件人: tricia
字节数: 194925
--------------------------------------------------
日期: 05 12, 06
发件人: phil
字节数: 1048
--------------------------------------------------
日期: 05 13, 06
发件人: barb
字节数: 271
--------------------------------------------------
日期: 05 14, 06
发件人: gene
字节数: 2291
--------------------------------------------------
日期: 05 14, 06
发件人: phil
字节数: 5781
--------------------------------------------------
日期: 05 14, 06
发件人: barb
字节数: 98151
--------------------------------------------------
日期: 05 14, 06
发件人: tricia
字节数: 2394482
--------------------------------------------------
日期: 05 15, 06
发件人: gene
字节数: 3824
--------------------------------------------------
日期: 05 15, 06
发件人: phil
字节数: 978
--------------------------------------------------
日期: 05 15, 06
发件人: gene
字节数: 998532
--------------------------------------------------
日期: 05 15, 06
发件人: gene
字节数: 3856
--------------------------------------------------
日期: 05 16, 06
发件人: gene
字节数: 613
--------------------------------------------------
日期: 05 16, 06
发件人: phil
字节数: 10294
--------------------------------------------------
日期: 05 17, 06
发件人: phil
字节数: 873
--------------------------------------------------
日期: 05 19, 06
发件人: gene
字节数: 23992
--------------------------------------------------
MySQL连接已关闭
如果要对这个数据进行更多分析，我们可以轻松修改查询，比如：
python
# 可以添加这样的分析查询
analysis_query = """
SELECT srcuser AS '发件人',COUNT(*) AS '邮件数量',SUM(size) AS '总字节数',AVG(size) AS '平均大小'
FROM mail
GROUP BY srcuser
ORDER BY COUNT(*) DESC
"""
3.5 合并多列来构建复合值
mysql> SELECT-> DATE_FORMAT(t, '%m %e, %y') AS 'date_sent',-> CONCAT(srcuser, '@', srchost) AS 'sender',-> CONCAT(dstuser, '@', dsthost) AS 'recipient',-> size FROM mail;
+-----------+---------------+---------------+---------+
| date_sent | sender | recipient | size |
+-----------+---------------+---------------+---------+
| 05 11, 06 | barb@saturn | tricia@mars | 58274 |
| 05 12, 06 | tricia@mars | gene@venus | 194925 |
| 05 12, 06 | phil@mars | phil@saturn | 1048 |
| 05 13, 06 | barb@saturn | tricia@venus | 271 |
| 05 14, 06 | gene@venus | barb@mars | 2291 |
| 05 14, 06 | phil@mars | tricia@saturn | 5781 |
| 05 14, 06 | barb@venus | barb@venus | 98151 |
| 05 14, 06 | tricia@saturn | phil@venus | 2394482 |
| 05 15, 06 | gene@mars | gene@saturn | 3824 |
| 05 15, 06 | phil@venus | phil@venus | 978 |
| 05 15, 06 | gene@mars | tricia@saturn | 998532 |
| 05 15, 06 | gene@saturn | gene@mars | 3856 |
| 05 16, 06 | gene@venus | barb@mars | 613 |
| 05 16, 06 | phil@venus | barb@venus | 10294 |
| 05 17, 06 | phil@mars | tricia@saturn | 873 |
| 05 19, 06 | gene@saturn | gene@venus | 23992 |
+-----------+---------------+---------------+---------+
16 rows in set (0.00 sec)
3.6 where表达式中的列别名
mysql> select t, srcuser, dstuser, size/1024 as kilobytes from mail where kilobytes > 500;
ERROR 1054 (42S22): Unknown column 'kilobytes' in 'where clause'mysql> select t, srcuser, dstuser, size/1024 as kilobytes from mail where size/1024 > 500;
+---------------------+---------+---------+-----------+
| t | srcuser | dstuser | kilobytes |
+---------------------+---------+---------+-----------+
| 2006-05-14 17:03:01 | tricia | phil | 2338.3613 |
| 2006-05-15 10:25:52 | gene | tricia | 975.1289 |
+---------------------+---------+---------+-----------+
2 rows in set (0.01 sec)
3.7 调试比较表达式
将在where 语句中设定的查询条件和查询结果一起显示给用户
mysql> select * from mail where srcuser < 'c' and size > 5000;
+---------------------+---------+---------+---------+---------+-------+
| t | srcuser | srchost | dstuser | dsthost | size |
+---------------------+---------+---------+---------+---------+-------+
| 2006-05-11 10:15:08 | barb | saturn | tricia | mars | 58274 |
| 2006-05-14 14:42:21 | barb | venus | barb | venus | 98151 |
+---------------------+---------+---------+---------+---------+-------+
2 rows in set (0.00 sec)mysql> select srcuser, srcuser < 'c', size, size > 5000 from mail;
+---------+---------------+---------+-------------+
| srcuser | srcuser < 'c' | size | size > 5000 |
+---------+---------------+---------+-------------+
| barb | 1 | 58274 | 1 |
| tricia | 0 | 194925 | 1 |
| phil | 0 | 1048 | 0 |
| barb | 1 | 271 | 0 |
| gene | 0 | 2291 | 0 |
| phil | 0 | 5781 | 1 |
| barb | 1 | 98151 | 1 |
| tricia | 0 | 2394482 | 1 |
| gene | 0 | 3824 | 0 |
| phil | 0 | 978 | 0 |
| gene | 0 | 998532 | 1 |
| gene | 0 | 3856 | 0 |
| gene | 0 | 613 | 0 |
| phil | 0 | 10294 | 1 |
| phil | 0 | 873 | 0 |
| gene | 0 | 23992 | 1 |
+---------+---------------+---------+-------------+
16 rows in set (0.00 sec)
3.8 使查询结果唯一化
在查询语句中加入distinct 关键字， 过滤冗余信息
mysql> select srcuser from mail;
+---------+
| srcuser |
+---------+
| barb |
| tricia |
| phil |
| barb |
| gene |
| phil |
| barb |
| tricia |
| gene |
| phil |
| gene |
| gene |
| gene |
| phil |
| phil |
| gene |
+---------+
16 rows in set (0.00 sec)mysql> select distinct srcuser from mail;
+---------+
| srcuser |
+---------+
| barb |
| tricia |
| phil |
| gene |
+---------+
4 rows in set (0.01 sec)mysql> select distinct year(t), month(t), dayofmonth(t) from mail;
+---------+----------+---------------+
| year(t) | month(t) | dayofmonth(t) |
+---------+----------+---------------+
| 2006 | 5 | 11 |
| 2006 | 5 | 12 |
| 2006 | 5 | 13 |
| 2006 | 5 | 14 |
| 2006 | 5 | 15 |
| 2006 | 5 | 16 |
| 2006 | 5 | 17 |
| 2006 | 5 | 19 |
+---------+----------+---------------+
8 rows in set (0.01 sec)mysql> select count(distinct srcuser) from mail;
+-------------------------+
| count(distinct srcuser) |
+-------------------------+
| 4 |
+-------------------------+
1 row in set (0.01 sec)
3.9 如何处理null 值
mysql> select * from taxpayer;
+---------+--------+
| name | id |
+---------+--------+
| bernina | 198-48 |
| bertha | NULL |
| ben | NULL |
| NULL | 475-83 |
| baidu | 111+55 |
+---------+--------+
5 rows in set (0.02 sec)mysql> select * from taxpayer where id = NULL;
Empty set (0.00 sec)mysql> select * from taxpayer where id != NULL;
Empty set (0.00 sec)mysql> select * from taxpayer where id is NULL;
+--------+------+
| name | id |
+--------+------+
| bertha | NULL |
| ben | NULL |
+--------+------+
2 rows in set (0.00 sec)mysql> select * from taxpayer where id is not NULL;
+---------+--------+
| name | id |
+---------+--------+
| bernina | 198-48 |
| NULL | 475-83 |
| baidu | 111+55 |
+---------+--------+
3 rows in set (0.00 sec)mysql> select NULL = NULL, NULL <=> NULL;
+-------------+---------------+
| NULL = NULL | NULL <=> NULL |
+-------------+---------------+
| NULL | 1 |
+-------------+---------------+
1 row in set (0.00 sec)mysql> select name, if(id is NULL, 'Unknown', id) as 'id' from taxpayer;
+---------+---------+
| name | id |
+---------+---------+
| bernina | 198-48 |
| bertha | Unknown |
| ben | Unknown |
| NULL | 475-83 |
| baidu | 111+55 |
+---------+---------+
5 rows in set (0.00 sec)mysql> select name, IFNULL(id , 'Unknown') as 'id' from taxpayer;
+---------+---------+
| name | id |
+---------+---------+
| bernina | 198-48 |
| bertha | Unknown |
| ben | Unknown |
| NULL | 475-83 |
| baidu | 111+55 |
+---------+---------+
5 rows in set (0.00 sec)