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HomeDatabaseMysql TutorialMysql字符串截取函数SUBSTRING的用法说明

今天建视图时,用到了MySQL中的字符串截取,很是方便。

感觉上MySQL的字符串函数截取字符,比用程序截取(如PHP或JAVA)来得强大,所以在这里做一个记录,希望对大家有用。
函数:
1、从左开始截取字符串
left(str, length)
说明:left(被截取字段,截取长度)
例:select left(content,200) as abstract from my_content_t
2、从右开始截取字符串
right(str, length)
说明:right(被截取字段,截取长度)
例:select right(content,200) as abstract from my_content_t
3、截取字符串
substring(str, pos)
substring(str, pos, length)
说明:substring(被截取字段,从第几位开始截取)
substring(被截取字段,从第几位开始截取,截取长度)
例:select substring(content,5) as abstract from my_content_t
select substring(content,5,200) as abstract from my_content_t
(注:如果位数是负数 如-5 则是从后倒数位数,到字符串结束或截取的长度)
4、按关键字截取字符串
substring_index(str,delim,count)
说明:substring_index(被截取字段,关键字,关键字出现的次数)
例:select substring_index("blog.jb51.net","。",2) as abstract from my_content_t
结果:blog.jb51
(注:如果关键字出现的次数是负数 如-2 则是从后倒数,到字符串结束)

函数简介:

SUBSTRING(str,pos) , SUBSTRING(str FROM pos) SUBSTRING(str,pos,len) , SUBSTRING(str FROM pos FOR len)

不带有len 参数的格式从字符串str返回一个子字符串,起始于位置 pos。带有len参数的格式从字符串str返回一个长度同len字符相同的子字符串,起始于位置 pos。 使用 FROM的格式为标准 SQL 语法。也可能对pos使用一个负值。假若这样,则子字符串的位置起始于字符串结尾的pos 字符,而不是字符串的开头位置。在以下格式的函数中可以对pos 使用一个负值。

详情请查阅手册。

实例:
表1:user

表2:jl

期望效果:通过user表jlid字段存储的id值,读取jl表中的相应记录,这里想要读取,jl表中id为1、2的记录,首先想到用in,但是很遗憾由于
jlid字段存储的id值有2个,尽管从形式上符合in(1,2)的格式,但是如果你使用select jl.* from jl where jl.id in(select jlid from user where user.id=1)来查询的话,是不行的,他总是返回id为1的记录。

那么怎么办呢?如果我们能够分别得到1,2中的1和2就行了。好在mysql也提供了字符串截取函数SUBSTRING。

sql句法如下:
SELECT jl. *
FROM jl
WHERE jl.id = (
SELECT SUBSTRING( (

SELECT user.jlid
FROM user
WHERE user.id =1
), 1, 1 ) )
OR jl.id = (

SELECT SUBSTRING( (

SELECT user.jlid
FROM user
WHERE user.id =1
), 3, 1 )
)
LIMIT 0 , 30

简单解释一下:

SELECT SUBSTRING( (SELECT user.jlid FROM user WHERE user.id =1), 1, 1 ) )
这里用了子查询,首先查询user表中,id为1的jlid字段的值,返回的是字符串,然后使用SUBSTRING进行截取,得到字符串1
SELECT SUBSTRING( (SELECT user.jlid FROM user WHERE user.id =1), 3, 1 ) )
这条语句得到2

1和2都得到了再通过主查询的where来查询,要注意我们需要查询id=1和id=2的记录,所以用到了OR,怎么样,是不是有点麻烦,

您的第一直觉是不是要用2条sql语句,中间再配合php的explode函数来查询呢?这样想是正常的,但是这两者之间谁的效率高,站长并没有测试,希望有心的你,可以帮忙哦!

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