甲骨文：

select substr(substr(MyField,1,20), 1, instr(substr(MyField,1,20), ' ',-1,1))
from MyTable

SQL服务器

SELECT LEFT(MyField, 20 - CHARINDEX (' ' ,REVERSE(LEFT(MyField,20))))
FROM MyTable

对于Oracle（应该可以将其转换为SQLServer，但我不知道SQLServer：

• 如果第一个“标记”（在第一个空格之前）超过20个字符，那么这将返回NULL
• 如果第21个字符是空格，则返回前20个字符
• 如果第21个字符不是空格，但前20个字符中有空格，则砍掉前20个字符，然后找到“最后”个空格并删除它和之后的所有内容。
• 如果整个字符串最多为20个字符，则应按原样返回。

在下面的测试数据中，我又添加了两个示例来测试这是否按需工作。

with
     inputs ( str ) as (
       select 'Lorem ipsum dolor sit amet, consectetur adipiscing elit' from dual union all
       select 'Lorem ipsumdolor sit amet, consectetur adipiscing elit'  from dual union all
       select 'Loremipsumdolorsitametconsedtetur'                       from dual union all
       select 'Lorem ipsumdolorsit amet, consectetur etc.'              from dual union all
       select 'Lorem ipsum dolorsitamet, consectetur adipiscing elit'   from dual union all
       select 'abcdef ghijk lmno'                                       from dual
     ),
     prep ( str, flag, fragment ) as (
       select str, 
              case when length(str) <= 20 or substr(str, 21, 1) = ' ' then 1 end, 
              substr(str, 1, 20)
       from   inputs
     )
select str, 
       case flag when 1 then fragment 
                        else substr(fragment, 1, instr(fragment, ' ', -1) - 1) end
       as new_str
from   prep;

STR                                                     NEW_STR            
------------------------------------------------------- --------------------
Lorem ipsum dolor sit amet, consectetur adipiscing elit Lorem ipsum dolor   
Lorem ipsumdolor sit amet, consectetur adipiscing elit  Lorem ipsumdolor sit
Loremipsumdolorsitametconsedtetur                                           
Lorem ipsumdolorsit amet, consectetur etc.              Lorem ipsumdolorsit 
Lorem ipsum dolorsitamet, consectetur adipiscing elit   Lorem ipsum
abcdef ghijk lmno                                       abcdef ghijk lmno 

如果SQL服务器，如果你不介意UDF。

Declare @YourTable table (SomeText varchar(500))
Insert Into @YourTable values
('Lorem ipsum dolor sit amet, consectetur adipiscing elit.'),
('Lorem ipsumdolor sit amet, consectetur adipiscing elit'),
('Lorem ipsum dolorsitamet, consectetur adipiscing elit')

Declare @MaxLen int = 20
Select *,Trimmed = [dbo].[udf-Str-TrimToWord](SomeText,@MaxLen)
 From  @YourTable

返回

SomeText                                                    Trimmed
Lorem ipsum dolor sit amet, consectetur adipiscing elit.    Lorem ipsum dolor
Lorem ipsumdolor sit amet, consectetur adipiscing elit      Lorem ipsumdolor
Lorem ipsum dolorsitamet, consectetur adipiscing elit       Lorem ipsum

那个UDF

CREATE FUNCTION [dbo].[udf-Str-TrimToWord] (@String varchar(max),@MaxLen int)
Returns varchar(max)
AS
Begin
    Return LEFT(@String,@MaxLen-CharIndex(' ' ,Reverse(Left(@String,@MaxLen))))
End