作用：解决 记得函数，不记得语法的窘境
查看总的目录
mysql> ? contents
//其下的是各大分类
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
比如 ，函数
mysql> ? functions
//其下的分类
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
再接着来
mysql> ? string functions
//截一段
You asked for help about help category: "String Functions"
For more information, type 'help <item>', where <item> is one of the following
topics:
ASCII
BIN
BINARY OPERATOR
BIT_LENGTH
CAST
CHAR FUNCTION
CHARACTER_LENGTH
CHAR_LENGTH
CONCAT
CONCAT_WS
CONVERT
ELT
EXPORT_SET
EXTRACTVALUE
接着即是相应的说明
mysql> ? convert
Name: 'CONVERT'
Description:
Syntax:
CONVERT(expr,type), CONVERT(expr USING transcoding_name)
The CONVERT() and CAST() functions take an expression of any type and
produce a result value of a specified type.
CAST() and CONVERT(... USING ...) are standard SQL syntax. The
non-USING form of CONVERT() is ODBC syntax.
CONVERT() with USING converts data between different character sets. In
MySQL, transcoding names are the same as the corresponding character
set names. For example, this statement converts the string 'abc' in the
default character set to the corresponding string in the utf8 character
set:
SELECT CONVERT('abc' USING utf8)
The type for the result can be one of the following values:
o BINARY[(N)]
o CHAR[(N)]
o DATE
o DATETIME
o DECIMAL[(M[,D])]
o SIGNED [INTEGER]
o TIME
o UNSIGNED [INTEGER]
BINARY produces a string with the BINARY data type. See
http://dev.mysql.com/doc/refman/5.5/en/binary-varbinary.html for a
description of how this affects comparisons. If the optional length N
is given, BINARY(N) causes the cast to use no more than N bytes of the
argument. Values shorter than N bytes are padded with 0x00 bytes to a
length of N.
CHAR(N) causes the cast to use no more than N characters of the
argument.
URL: http://dev.mysql.com/doc/refman/5.5/en/cast-functions.html
Examples:
SELECT enum_col FROM tbl_name ORDER BY CAST(enum_col AS CHAR)
所以，简单来说，比如不知道 date怎么用，就是
? date
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