曹县网站建设公司常州百度关键词优化
4.3 使用临时表
Drop table 语句来删除表，
选择使用create temporary table 语句，创建的是一张临时表。
Create temporary table tb1_name(…列定义…)
克隆表
Create temporary table new_table like original_table
根据查询结果建表
Create temporary table tb1_name select …
临时表可以使用普通表的表名。在临时表的生命周期内，它将屏蔽与之同名的普通表。对其任意操作，不影响真实数据。
mysql> create temporary table mail select * from mail;
Query OK, 16 rows affected (0.02 sec)
Records: 16 Duplicates: 0 Warnings: 0mysql> select count(*) from mail;
+----------+
| count(*) |
+----------+
| 16 |
+----------+
1 row in set (0.01 sec)mysql> delete from mail;
Query OK, 16 rows affected (0.00 sec)mysql> select count(*) from mail;
+----------+
| count(*) |
+----------+
| 0 |
+----------+
1 row in set (0.00 sec)mysql> drop table mail;
Query OK, 0 rows affected (0.00 sec)mysql> select count(*) from mail;
+----------+
| count(*) |
+----------+
| 16 |
+----------+
1 row in set (0.00 sec)
只有在最后一次使用之后，数据库才会自动删除临时表
应用程序中使用一个与普通表同名的临时表时， 应用程序只会对该临时表就行修改。
应用程序API提供数据库的持久连接或者连接池， 可以使用这些技术保证临时表的持久性。
比较明智的做法是在创建临时表之前执行下面的语句
Drop temporary table if exists tb1_name
4.4 检查或改变某个表的存储引擎
mysql> SELECT engine FROM information_schema.tables-> WHERE table_schema = 'cookbook' AND table_name = 'mail';
+--------+
| ENGINE |
+--------+
| InnoDB |
+--------+
1 row in set (0.02 sec)
mysql> show table status like 'mail'\G
*************************** 1. row ***************************Name: mailEngine: InnoDBVersion: 10Row_format: DynamicRows: 16Avg_row_length: 1024Data_length: 16384
Max_data_length: 0Index_length: 16384Data_free: 0Auto_increment: NULLCreate_time: 2024-12-17 19:34:31Update_time: NULLCheck_time: NULLCollation: utf8mb4_0900_ai_ciChecksum: NULLCreate_options:Comment:
1 row in set (0.01 sec)
mysql> show create table mail\G
*************************** 1. row ***************************Table: mail
Create Table: CREATE TABLE `mail` (`t` datetime DEFAULT NULL,`srcuser` char(8) DEFAULT NULL,`srchost` char(20) DEFAULT NULL,`dstuser` char(8) DEFAULT NULL,`dsthost` char(20) DEFAULT NULL,`size` bigint DEFAULT NULL,KEY `t` (`t`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci
1 row in set (0.00 sec)
4.5 生成唯一的表名
如果你能创建一张temporary表， 就无所谓表名是否重复了。
Import io
Tb1_name = “tmp_tb1_%d” % os.getpid()
https://zhuanlan.zhihu.com/p/721613208
https://zhuanlan.zhihu.com/p/367182300
https://www.zhihu.com/column/c_1253693093662093312
https://www.zhihu.com/question/55720139/answer/1719997534
from multiprocessing import Process
import os# 子进程要执行的代码
def run_proc(name):print('Run child proces: %s (%s)...' % (name, os.getpid()))if __name__ == '__main__':print('Parent process %s.' % os.getpid())p = Process(target=run_proc, args=('test', ))print('Child process will start.')p.start()p.join()
print('Child process end.')
Parent process 15256.
Child process will start.
Child process end.
``
import os
from multiprocessing import Processdef child_process():print('I am child process (%s) and my parent is %s.' % (os.getpid(), os.getppid()))if __name__ == '__main__':print('Process (%s) start...' % os.getpid())p = Process(target=child_process)p.start()p.join()print('I (%s) just created a child process (%s).' % (os.getpid(), p.pid))Process (15256) start...
I (15256) just created a child process (13264).import os
import sys
from multiprocessing import Processdef child_process():# Force immediate output (critical for Windows)sys.stdout.flush()sys.stderr.flush()print(f'CHILD: I am process {os.getpid()} with parent {os.getppid()}')# Explicitly flush againsys.stdout.flush()if __name__ == '__main__':print(f'PARENT: Process {os.getpid()} starting...')p = Process(target=child_process)p.start()# Wait for child to finishp.join() print(f'PARENT: Created child process {p.pid}')print("Note: If you don't see CHILD output above, try running this in:")print("1. A terminal (not IDE)")
print("2. Command Prompt: python script.py")PARENT: Process 15256 starting...
PARENT: Created child process 1044
Note: If you don't see CHILD output above, try running this in:
1. A terminal (not IDE)
2. Command Prompt: python script.py
使用下面的语句安全的删除它：
Drop table if exists tb1_name
第5章：与字符串共舞
5.0 引言
5.1 字符串属性
mysql> show character set;
+----------+---------------------------------+---------------------+--------+
| Charset | Description | Default collation | Maxlen |
+----------+---------------------------------+---------------------+--------+
| armscii8 | ARMSCII-8 Armenian | armscii8_general_ci | 1 |
| ascii | US ASCII | ascii_general_ci | 1 |
| big5 | Big5 Traditional Chinese | big5_chinese_ci | 2 |
| binary | Binary pseudo charset | binary | 1 |
| cp1250 | Windows Central European | cp1250_general_ci | 1 |
| cp1251 | Windows Cyrillic | cp1251_general_ci | 1 |
| cp1256 | Windows Arabic | cp1256_general_ci | 1 |
| cp1257 | Windows Baltic | cp1257_general_ci | 1 |
| cp850 | DOS West European | cp850_general_ci | 1 |
| cp852 | DOS Central European | cp852_general_ci | 1 |
| cp866 | DOS Russian | cp866_general_ci | 1 |
| cp932 | SJIS for Windows Japanese | cp932_japanese_ci | 2 |
| dec8 | DEC West European | dec8_swedish_ci | 1 |
| eucjpms | UJIS for Windows Japanese | eucjpms_japanese_ci | 3 |
| euckr | EUC-KR Korean | euckr_korean_ci | 2 |
| gb18030 | China National Standard GB18030 | gb18030_chinese_ci | 4 |
| gb2312 | GB2312 Simplified Chinese | gb2312_chinese_ci | 2 |
| gbk | GBK Simplified Chinese | gbk_chinese_ci | 2 |
| geostd8 | GEOSTD8 Georgian | geostd8_general_ci | 1 |
| greek | ISO 8859-7 Greek | greek_general_ci | 1 |
| hebrew | ISO 8859-8 Hebrew | hebrew_general_ci | 1 |
| hp8 | HP West European | hp8_english_ci | 1 |
| keybcs2 | DOS Kamenicky Czech-Slovak | keybcs2_general_ci | 1 |
| koi8r | KOI8-R Relcom Russian | koi8r_general_ci | 1 |
| koi8u | KOI8-U Ukrainian | koi8u_general_ci | 1 |
| latin1 | cp1252 West European | latin1_swedish_ci | 1 |
| latin2 | ISO 8859-2 Central European | latin2_general_ci | 1 |
| latin5 | ISO 8859-9 Turkish | latin5_turkish_ci | 1 |
| latin7 | ISO 8859-13 Baltic | latin7_general_ci | 1 |
| macce | Mac Central European | macce_general_ci | 1 |
| macroman | Mac West European | macroman_general_ci | 1 |
| sjis | Shift-JIS Japanese | sjis_japanese_ci | 2 |
| swe7 | 7bit Swedish | swe7_swedish_ci | 1 |
| tis620 | TIS620 Thai | tis620_thai_ci | 1 |
| ucs2 | UCS-2 Unicode | ucs2_general_ci | 2 |
| ujis | EUC-JP Japanese | ujis_japanese_ci | 3 |
| utf16 | UTF-16 Unicode | utf16_general_ci | 4 |
| utf16le | UTF-16LE Unicode | utf16le_general_ci | 4 |
| utf32 | UTF-32 Unicode | utf32_general_ci | 4 |
| utf8mb3 | UTF-8 Unicode | utf8mb3_general_ci | 3 |
| utf8mb4 | UTF-8 Unicode | utf8mb4_0900_ai_ci | 4 |
+----------+---------------------------------+---------------------+--------+
41 rows in set (0.01 sec)
mysql> set @s = convert('abc' using ucs2);
Query OK, 0 rows affected, 1 warning (0.00 sec)mysql> select length(@s), char_length(@s);
+------------+-----------------+
| length(@s) | char_length(@s) |
+------------+-----------------+
| 6 | 3 |
+------------+-----------------+
1 row in set (0.00 sec)mysql> set @s = convert('abc' using utf8);
Query OK, 0 rows affected, 1 warning (0.00 sec)mysql> select length(@s), char_length(@s);
+------------+-----------------+
| length(@s) | char_length(@s) |
+------------+-----------------+
| 3 | 3 |
+------------+-----------------+
1 row in set (0.00 sec)
mysql> show collation like 'latin1%';
+-------------------+---------+----+---------+----------+---------+---------------+
| Collation | Charset | Id | Default | Compiled | Sortlen | Pad_attribute |
+-------------------+---------+----+---------+----------+---------+---------------+
| latin1_bin | latin1 | 47 | | Yes | 1 | PAD SPACE |
| latin1_danish_ci | latin1 | 15 | | Yes | 1 | PAD SPACE |
| latin1_general_ci | latin1 | 48 | | Yes | 1 | PAD SPACE |
| latin1_general_cs | latin1 | 49 | | Yes | 1 | PAD SPACE |
| latin1_german1_ci | latin1 | 5 | | Yes | 1 | PAD SPACE |
| latin1_german2_ci | latin1 | 31 | | Yes | 2 | PAD SPACE |
| latin1_spanish_ci | latin1 | 94 | | Yes | 1 | PAD SPACE |
| latin1_swedish_ci | latin1 | 8 | Yes | Yes | 1 | PAD SPACE |
+-------------------+---------+----+---------+----------+---------+---------------+
8 rows in set (0.01 sec)
mysql> create table t (c char(3) character set latin1);
Query OK, 0 rows affected (0.12 sec)mysql> insert into t (c) values('AAA'),('aaa'),('bbb'),('BBB');
Query OK, 4 rows affected (0.02 sec)
Records: 4 Duplicates: 0 Warnings: 0mysql> select c from t;
+------+
| c |
+------+
| AAA |
| aaa |
| bbb |
| BBB |
+------+
4 rows in set (0.00 sec)mysql> select c from t order by c collate latin1_swedish_ci;
+------+
| c |
+------+
| AAA |
| aaa |
| bbb |
| BBB |
+------+
4 rows in set (0.00 sec)mysql> select c from t order by c collate latin1_general_cs;
+------+
| c |
+------+
| AAA |
| aaa |
| BBB |
| bbb |
+------+
4 rows in set (0.00 sec)mysql> select c from t order by c collate latin1_bin;
+------+
| c |
+------+
| AAA |
| BBB |
| aaa |
| bbb |
+------+
4 rows in set (0.00 sec)mysql> create table tt (c char(2) character set utf8);
Query OK, 0 rows affected, 1 warning (0.06 sec)mysql> insert into tt(c) values('cg'), ('ch'), ('ci'), ('lk'), ('ll'), ('lm');
Query OK, 6 rows affected (0.01 sec)
Records: 6 Duplicates: 0 Warnings: 0mysql> select c from tt order by c collate utf8_general_ci;
+------+
| c |
+------+
| cg |
| ch |
| ci |
| lk |
| ll |
| lm |
+------+
6 rows in set (0.00 sec)mysql> select c from tt order by c collate utf8_spanish2_ci;
+------+
| c |
+------+
| cg |
| ci |
| ch |
| lk |
| lm |
| ll |
+------+
6 rows in set (0.00 sec)5.2 选择字符串的数据类型
`mysql> create table t2 (c1 char(10), c2 varchar(10));
Query OK, 0 rows affected (0.06 sec)mysql> insert into t2(c1, c2) values('abc ', 'abc '); #用2个tab 键
Query OK, 1 row affected (0.01 sec)mysql> select c1, c2, char_length(c1), char_length(c2) from t2;
+------+-------+-----------------+-----------------+
| c1 | c2 | char_length(c1) | char_length(c2) |
+------+-------+-----------------+-----------------+
| abc | abc | 4 | 5 |
+------+-------+-----------------+-----------------+
1 row in set (0.00 sec)mysql> insert into t2(c1, c2) values('abc ', 'abc '); #用5个空格
Query OK, 1 row affected (0.01 sec)mysql> select c1, c2, char_length(c1), char_length(c2) from t2;
+------+----------+-----------------+-----------------+
| c1 | c2 | char_length(c1) | char_length(c2) |
+------+----------+-----------------+-----------------+
| abc | abc | 4 | 5 |
| abc | abc | 3 | 8 |
+------+----------+-----------------+-----------------+
2 rows in set (0.00 sec)
5.3 正确设置客户端连接的字符集
5.4 串字母
mysql> select 'I''m asleep', 'I\'m wide awake'-> ;
+------------+----------------+
| I'm asleep | I'm wide awake |
+------------+----------------+
| I'm asleep | I'm wide awake |
+------------+----------------+
1 row in set (0.00 sec)mysql> select "He said, ""Boo!""", "And I said, \"Yikee!\"";
+-----------------+----------------------+
| He said, "Boo!" | And I said, "Yikee!" |
+-----------------+----------------------+
| He said, "Boo!" | And I said, "Yikee!" |
+-----------------+----------------------+
1 row in set (0.00 sec)mysql> select 'Install MySql in C:\\mysql on windows';
+--------------------------------------+
| Install MySql in C:\mysql on windows |
+--------------------------------------+
| Install MySql in C:\mysql on windows |
+--------------------------------------+
1 row in set (0.00 sec)MySQL 将 0x 前缀的值解释为二进制字面量，直接查询时不会自动转换为字符串：
SELECT 0x49276D2061736C656570; -- 显示二进制原值
+------------------------------------------------+
| 0x49276D2061736C656570 |
+------------------------------------------------+
| 0x49276D2061736C656570 |
+------------------------------------------------+
1 row in set (0.00 sec)
若需强制显示为字符串，需显式转换：
mysql> SELECT CONCAT('0x', HEX('I''m asleep')) AS hex_result;
+------------------------+
| hex_result |
+------------------------+
| 0x49276D2061736C656570 |
+------------------------+
1 row in set (0.00 sec)mysql> SELECT CAST(0x49276D2061736C656570 AS CHAR) AS original_string;
+-----------------+
| original_string |
+-----------------+
| I'm asleep |
+-----------------+
1 row in set (0.00 sec)mysql> SELECT CONVERT(UNHEX('49276D2061736C656570') USING utf8) AS original_string;
+-----------------+
| original_string |
+-----------------+
| I'm asleep |
+-----------------+
1 row in set, 1 warning (0.00 sec)5.5 检查一个字符串的字符集或字符排序
mysql> CREATE TABLE t (-> c CHAR(10)-> ) CHARACTER SET utf8 COLLATE utf8_danish_ci;
Query OK, 0 rows affected, 2 warnings (0.34 sec)mysql> select user(), charset(user()), collation(user());
+----------------+-----------------+--------------------+
| user() | charset(user()) | collation(user()) |
+----------------+-----------------+--------------------+
| root@localhost | utf8mb3 | utf8mb3_general_ci |
+----------------+-----------------+--------------------+
1 row in set (0.01 sec)mysql> set names-> 'latin1';
Query OK, 0 rows affected (0.01 sec)mysql> select charset('abc'), collation('abc');
+----------------+-------------------+
| charset('abc') | collation('abc') |
+----------------+-------------------+
| latin1 | latin1_swedish_ci |
+----------------+-------------------+
1 row in set (0.00 sec)mysql> set names latin7 collate 'latin7_bin';
Query OK, 0 rows affected (0.00 sec)mysql> select charset('abc'), collation('abc');
+----------------+------------------+
| charset('abc') | collation('abc') |
+----------------+------------------+
| latin7 | latin7_bin |
+----------------+------------------+
1 row in set (0.00 sec)
- 为什么第一个查询显示 latin7？
o 这是由系统变量 character_set_connection 决定的
o 可以查看当前字符集设置：
sql
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SHOW VARIABLES LIKE 'character_set%';
SHOW VARIABLES LIKE 'collation%';
2. 如何确保使用 UTF-8 编码？
sql
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-- 设置连接字符集
SET NAMES utf8mb4;-- 再次检查
SELECT CHARSET('a'), COLLATION('a');
3. 创建表时指定字符集（最佳实践）
sql
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CREATE TABLE t (c CHAR(10)
) CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;
示例完整流程
sql
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-- 1. 创建表（使用明确字符集）
CREATE TABLE t (c CHAR(10)
) CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;-- 2. 插入测试数据
INSERT INTO t VALUES ('test');-- 3. 查询列字符集信息
SELECT CHARSET(c) AS column_charset,COLLATION(c) AS column_collation
FROM t
LIMIT 1;-- 4. 查询表结构元数据
SELECT column_name,character_set_name,collation_name
FROM information_schema.columns
WHERE table_schema = DATABASE()AND table_name = 't';
输出
+----------------+-------------------+
| column_charset | column_collation |
+----------------+-------------------+
| utf8mb4 | utf8mb4_unicode_ci|
+----------------+-------------------++-------------+--------------------+-------------------+
| COLUMN_NAME | CHARACTER_SET_NAME | COLLATION_NAME |
+-------------+--------------------+-------------------+
| c | utf8mb4 | utf8mb4_unicode_ci|
+-------------+--------------------+-------------------+mysql> CREATE TABLE t (-> c CHAR(10)-> ) character set utf8mb4 collate utf8mb4_unicode_ci;
Query OK, 0 rows affected (0.07 sec)mysql> SELECT-> CHARSET(c) AS column_charset,-> COLLATION(c) AS column_collation-> FROM-> t;
Empty set (0.00 sec)mysql>
mysql> SELECT-> column_name,-> character_set_name,-> collation_name-> FROM-> information_schema.columns-> WHERE-> table_schema = DATABASE()-> AND table_name = 't';
+-------------+--------------------+--------------------+
| COLUMN_NAME | CHARACTER_SET_NAME | COLLATION_NAME |
+-------------+--------------------+--------------------+
| c | utf8mb4 | utf8mb4_unicode_ci |
+-------------+--------------------+--------------------+
1 row in set (0.00 sec)mysql> alter table t-> convert to character set utf8mb4-> collate utf8mb4_unicode_ci;
Query OK, 0 rows affected (0.04 sec)
Records: 0 Duplicates: 0 Warnings: 0
关键区别说明
特性 utf8mb3 (旧版) utf8mb4 (推荐)
最大字符长度 3字节 4字节
支持emoji ❌ ✅
存储开销 较小 稍大
MySQL版本 所有版本 5.5.3+
为什么推荐utf8mb4？
完整支持所有Unicode字符（包括中文生僻字、emoji）
未来兼容性更好
MySQL 8.0默认使用utf8mb4