大家好，我是考100分的小小码 ，祝大家学习进步，加薪顺利呀。今天说一说mysql的一些命令_MySQL case when,希望您对编程的造诣更进一步.
这是MySQL系列笔记的第一部分，本系列笔记希望能按照笔者自己学习MySQL技术的经历来记录，避免纯粹按照内容一块一块总结，也就是不同于一般按内容分配章节的书籍的结构，有一个平滑的阅读曲线。内容比较丰富的技术点会按照专题在多个学习笔记中逐渐深入。
首先，请如果还完全为接触过MySQL的读者先阅读w3c的SQL教程，再来读本篇内容。
这部分的标题叫比CRUD多一丁点儿，比起最基础的w3c的SQL教程之外，只多一点的扩展，满足应付从纯粹阅读入门资料到可以上手完成一个简单的工作的需求。
第一篇的主要内容会是最常用的一些mysql命令。因为虽然有很多图形化的工具，但在实际的工作中因为需要去线上服务查看数据，处理问题，字符界面的命令行工具是必不可少的。
客户端程序mysql:
连接交互式终端
mysql -h $host -P $port -u $user -p$passsword $database_name
例如用户root使用密码mydb@xxx用链接到地址为192.168.1.99，端口为3306的数mysql进程，并默认使用上名为testdb的数据库（即自动执行use testdb）。
mysql -h 192.168.1.99 -P 3306 -u"root" -p"mydb@xx" testdb
各参数如果有@,&等bash的关键字，则需要用””引起来。
非交互式的执行SQL
有时候需要在命令行执行某句SQL，则建议使用 -Bse 参数。-B参数的含义是执行多条语句(batch)这样可以执行多条，-e即是执行(execute)，-s参数意思是静默执行(silent)可以让输出格式精简一些。
mysql -h $host -P $port -u $user -p$passsword -Bse "command1;command2;....;commandn"
例如，常见的将执行结果导出到文件中方便留存和阅读。
mysql -h 192.168.1.99 -P 3306 -u"root" -p"mydb@xx" -Bse "select id,name from testdb.Account;" > result.txt
导出程序mysqldump
mysqldump是做数据导出的命令行工具，也是mysql安装后默认会带的。作用是将mysql数据库中的数据导出出来。
导出特定的表的内容
mysql -h $host -P $port -u $user -p$passsword $database_name $table1 $table2 ...
```bash 例如 ```bash
mysqldump -h 192.168.1.99 -P 3306 -u"root" -p"mydb@xx" mydb table1 table2 > result.sql
只导出表结构
使用--no-data参数只导出表结构，
mysqldump -h 192.168.1.99 -P 3306 -u"root" -p"mydb@xx" --no-data mydb > result.sql
比较表结构mysqldiff
在开发实践中难免会遇到校验数据表结构不同，或者根据开发环境和目标环境的表结构不同来生成对应的表结构修改语句。mysql在5.7版本就提供了一个自带的mysqldiff工具。参数比较多，直接举例说明生成difftype=sql的ALTER语句的命令写法，如下：
mysqldiff --server1=root@host1 --server2=root@host2 \
--show-reverse --difftype=sql \
db1.table1:dbx.table3
# server1 on host1: ... connected.
# server2 on host2: ... connected.
# Comparing db1.table1 to dbx.table3 [FAIL]
# Transformation statements:
# --destination=server1:
ALTER TABLE db1.table1
ADD COLUMN notes char(30) AFTER a,
CHANGE COLUMN misc misc char(55);
# --destination=server2:
# ALTER TABLE dbx.table3
# DROP COLUMN notes,
# CHANGE COLUMN misc misc char(30);
具体看mysql的官方文档：https://dev.mysql.com/doc/mysql-utilities/1.5/en/mysqldiff.html
如果在mysql5.7版本以下就使用mysqldump命令参数输出简洁的表结构，随后diff文件然后自行编写ALTER语句吧。
mysqldump --skip-comments --skip-extended-insert -u root -p dbName1>file1.sql
mysqldump --skip-comments --skip-extended-insert -u root -p dbName2>file2.sql
diff file1.sql file2.sql
内建?, \s命令
心急火燎的进入mysql终端处理线上问题，这时候一个语法拼不对，还得切出去查手册？万一环境是内网不能上网就更是麻烦，其实mysql内建了帮助手册，可以直接在终端查询。
? 是交互式mysql终端内建的帮助命令。可以按照此帮助查阅文档等。其输出如下
mysql> ?
For information about MySQL products and services, visit:
http://www.mysql.com/
For developer information, including the MySQL Reference Manual, visit:
http://dev.mysql.com/
To buy MySQL Enterprise support, training, or other products, visit:
https://shop.mysql.com/
List of all MySQL commands:
Note that all text commands must be first on line and end with ';'
? (\?) Synonym for `help'.
clear (\c) Clear the current input statement.
connect (\r) Reconnect to the server. Optional arguments are db and host.
delimiter (\d) Set statement delimiter.
edit (\e) Edit command with $EDITOR.
ego (\G) Send command to mysql server, display result vertically.
exit (\q) Exit mysql. Same as quit.
go (\g) Send command to mysql server.
help (\h) Display this help.
nopager (\n) Disable pager, print to stdout.
notee (\t) Don't write into outfile.
pager (\P) Set PAGER [to_pager]. Print the query results via PAGER.
print (\p) Print current command.
prompt (\R) Change your mysql prompt.
quit (\q) Quit mysql.
rehash (\#) Rebuild completion hash.
source (\.) Execute an SQL script file. Takes a file name as an argument.
status (\s) Get status information from the server.
system (\!) Execute a system shell command.
tee (\T) Set outfile [to_outfile]. Append everything into given outfile.
use (\u) Use another database. Takes database name as argument.
charset (\C) Switch to another charset. Might be needed for processing binlog with multi-byte charsets.
warnings (\W) Show warnings after every statement.
nowarning (\w) Don't show warnings after every statement.
resetconnection(\x) Clean session context.
\s 可查看当前状态，版本，客户端ip，QPS等，\!可以在mysql终端中执行shell命令。在是很多处理问题的时候终端界面一进入mysql的交互式终端，就找不到ip、端口等在bash中的信息了又需要频繁切出，这两个命令都非常有用。\s输出如下：
mysql> \s
--------------
mysql Ver 14.14 Distrib 5.7.12, for osx10.11 (x86_64) using EditLine wrapper
Connection id: 2
Current database: rizhiyi_system
Current user: root@localhost
SSL: Not in use
Current pager: less
Using outfile: ''
Using delimiter: ;
Server version: 5.7.12 MySQL Community Server (GPL)
Protocol version: 10
Connection: 127.0.0.1 via TCP/IP
Server characterset: utf8
Db characterset: utf8
Client characterset: utf8
Conn. characterset: utf8
TCP port: 3306
Uptime: 21 sec
Threads: 1 Questions: 74 Slow queries: 0 Opens: 171 Flush tables: 1 Open tables: 164 Queries per second avg: 3.523
--------------
? contents 可以看内建的帮助手册
mysql> ? contents
You asked for help about help category: "Contents"
For more information, type 'help <item>', where <item> is one of the following
categories:
Account Management
Administration
Compound Statements
Data Definition
Data Manipulation
Data Types
Functions
Functions and Modifiers for Use with GROUP BY
Geographic Features
Help Metadata
Language Structure
Plugins
Procedures
Storage Engines
Table Maintenance
Transactions
User-Defined Functions
Utility
然后就可以继续查看子菜单帮助手册的内容，比如? Functions
mysql> ? Functions
You asked for help about help category: "Functions"
For more information, type 'help <item>', where <item> is one of the following
categories:
Bit Functions
Comparison operators
Control flow functions
Date and Time Functions
Encryption Functions
Information Functions
Logical operators
Miscellaneous Functions
Numeric Functions
String Functions
? insert 帮助命令还可以直接按关键字进行模糊查询，如：
mysql> ? insert
Name: 'INSERT'
Description:
Syntax:
INSERT [LOW_PRIORITY | DELAYED | HIGH_PRIORITY] [IGNORE]
[INTO] tbl_name
[PARTITION (partition_name,...)]
[(col_name,...)]
{VALUES | VALUE} ({expr | DEFAULT},...),(...),...
[ ON DUPLICATE KEY UPDATE
col_name=expr
[, col_name=expr] ... ]
Or:
INSERT [LOW_PRIORITY | DELAYED | HIGH_PRIORITY] [IGNORE]
[INTO] tbl_name
[PARTITION (partition_name,...)]
SET col_name={expr | DEFAULT}, ...
[ ON DUPLICATE KEY UPDATE
col_name=expr
[, col_name=expr] ... ]
Or:
INSERT [LOW_PRIORITY | HIGH_PRIORITY] [IGNORE]
[INTO] tbl_name
[PARTITION (partition_name,...)]
[(col_name,...)]
SELECT ...
[ ON DUPLICATE KEY UPDATE
col_name=expr
[, col_name=expr] ... ]
INSERT inserts new rows into an existing table. The INSERT ... VALUES
and INSERT ... SET forms of the statement insert rows based on
explicitly specified values. The INSERT ... SELECT form inserts rows
selected from another table or tables. INSERT ... SELECT is discussed
further in [HELP INSERT SELECT].
When inserting into a partitioned table, you can control which
partitions and subpartitions accept new rows. The PARTITION option
takes a comma-separated list of the names of one or more partitions or
subpartitions (or both) of the table. If any of the rows to be inserted
by a given INSERT statement do not match one of the partitions listed,
the INSERT statement fails with the error Found a row not matching the
given partition set. See
http://dev.mysql.com/doc/refman/5.7/en/partitioning-selection.html, for
more information and examples.
In MySQL 5.7, the DELAYED keyword is accepted but ignored by the
server. See [HELP INSERT DELAYED], for the reasons for this.
URL: http://dev.mysql.com/doc/refman/5.7/en/insert.html
本文中常用命令的更详细内容可看MySQL手册： dev.mysql.com/doc/refman/…
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