TRIM,LTRIMandRTRIMareSQLfunctions used to filter unwanted characters from strings. By default they remove spaces, but a set of characters can be specified for removal as well.

TRIM,?LTRIM?、RTRIM是用于過濾一些不想要的字符的sql函數(shù)，默認(rèn)去除空格，也可以去除指定的字符串。

TRIM enables you to trim leading or trailing characters (or both) from a character string. If trim_character or trim_source is a character literal, then you must enclose it in single quotes.

If you specify LEADING, then Oracle Database removes any leading characters equal to trim_character.

If you specify TRAILING, then Oracle removes any trailing characters equal to trim_character.

If you specify BOTH or none of the three, then Oracle removes leading and trailing characters equal to trim_character.

If you do not specify trim_character, then the default value is a blank space.

If you specify only trim_source, then Oracle removes leading and trailing blank spaces.

The function returns a value with datatype VARCHAR2. The maximum length of the value is the length of trim_source.

If either trim_source or trim_character is null, then the TRIM function returns null.

Both trim_character and trim_source can be any of the datatypes CHAR, VARCHAR2, NCHAR, NVARCHAR2, CLOB, or NCLOB. The string returned is of VARCHAR2 datatype if trim_source is a character datatype and a LOB if trim_source is a LOB datatype. The return string is in the same character set as trim_source.

下面舉例說明：

1、trim(str)等價(jià)于trim(both from '字符串')

select trim('? 去除前后空格? ') from dual;

等價(jià)于

SQL> select trim(both from'? 去除前后空格? ') from dual;

TRIM(BOTHFROM'去除前后空格')

----------------------------

去除前后空格

2、trim(leading from str)

SQL> select trim(leading from '? 用leading只去除前面空格，后面的空格不會去除? ')||'看看前面那句話后面的空格沒去掉吧' from dual;

TRIM(LEADINGFROM'用LEADING只去

-----------------------------------------------------------------------------

用leading只去除前面空格，后面的空格不會去除? 看看前面那句話后面的空格沒去掉吧

3、trim(trailing from '?字符串')

SQL> select 'Start'||trim(trailing from '? 用trailing只去除末尾的空格，字符串前面的空格不會去除? ')||'End' from dual;

'START'||TRIM(TRAILINGFROM'用T

--------------------------------------------------------------

Start? 用trailing只去除末尾的空格，字符串前面的空格不會去除End

4、刪除字符串末尾的字符d

SQL> select 'Start='||trim(trailing 'd' from 'Hello World')||'=End' from dual;

'START='||TRIM(TRAILING'D'FROM

------------------------------

Start=Hello Worl=End

LTRIM

LTRIM()(char [,set])：該函數(shù)用于去掉字符串char左端所包含的set中的任何字符。Oracle從左端第一個(gè)字符開始掃描，逐一去掉在set中出現(xiàn)的字符，當(dāng)遇到不是set中的字符時(shí)終止，然后返回剩余結(jié)果。

先看幾個(gè)實(shí)例：

SQL> select ltrim('109224323','109') from dual;

LTRIM('109224323','109')

------------------------

224323

這個(gè)的功能應(yīng)該都知道的噢~~? 再來看一個(gè)：

SQL> select ltrim('10900094323','109') from dual;

LTRIM('10900094323','109')

---------------------------

4323

是不是有點(diǎn)迷糊了？按道理說應(yīng)該是00094323的結(jié)果嘛~~? 再來看兩個(gè)對比的：

SQL> select ltrim('10900111000991110224323','109') from dual;

LTRIM('10900111000991110224323

------------------------------

224323

SQL> select ltrim('109200111000991110224323','109') from dual;

LTRIM('10920011100099111022432

------------------------------

200111000991110224323

是不是有這樣的疑問：為什么第二個(gè)查詢語句多了一個(gè)2就沒被截了呢？

再來看一個(gè)：

SQL> select ltrim('902100111000991110224323','109') from dual;

LTRIM('90210011100099111022432

------------------------------

2100111000991110224323

我想大家都都會想：按道理說是截109的值，為什么90也被截了？

總結(jié)：ltrim(x,y) 函數(shù)是按照y中的字符一個(gè)一個(gè)截掉x中的字符，并且是從左邊開始執(zhí)行的，只要遇到y(tǒng)中有的字符, x中的字符都會被截掉, 直到在x的字符中遇到y(tǒng)中沒有的字符為止函數(shù)命令才結(jié)束 .

函數(shù)將109當(dāng)成了三個(gè)字符以1，0，9在字符串開始直道出現(xiàn)不為1，0，9這三個(gè)字符中的任意一個(gè)開始截取;

可以看出，ltrim函數(shù)是從匹配函數(shù)開始之后出現(xiàn)在子串中任何字符都被屏蔽掉了;

ltrim('902100111000991110224323','109')從左邊開始截取，直到出現(xiàn)不符合1，0，9結(jié)束，即到2停止截取，不管2后面是否存在1，0，9照樣輸出，rtrim()同理

RTRIM 同LTRIM。。。

總結(jié)

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