Character interception triplets of data warehouse: substrb, substr, substring

Abstract ： Here are some usages and differences of these three functions in character interception .

This article is shared from Huawei cloud community 《​ ​GaussDB(DWS) The characters in are used to intercept triplets ​​》, author ： I stand under the northern sky .

stay GaussDB(DWS) There is a function triplet in support of character interception function in , They are substrb()、substr()、substring(), Many people probably only know that they can manipulate string interception , It may not be very clear if we go deeper , Some are parameter truncation length 、 Some parameters are end positions 、 Some parameters can be negative 、 Some can't accept negative parameters ·····
Let's introduce some usages and differences of these three functions in character interception .

summary

substr,substrb,substring Are string interception functions , Can take two or three parameters , Used to extract a character of a specified length starting at a specified position in a string . The function is defined as follows ：

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        Function form ：
       
       
    substrb(string, from [, count])
       
       
    substr(string, from [, count])
       
       
    substring(string, from [, count])
       
       
 Parameters to describe ：
       
       
     From parameter string Extract substring from ,from Indicates the starting position of the extraction ,count Represents the length of the extracted substring .
       
       
 return type ：
       
       
    text
      
      
      
      

       
       
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differences

1. Interception unit

substrb Intercept by byte ,substr/substring Intercept by character . With utf8 Coding, for example ,1 Chinese characters account for 3 Bytes , When using substrb Interception length 3 When you are in a string , Only one character can be intercepted , and substr/substring Three characters can be intercepted .

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       postgres=# select substrb('hwgs HUAWEI ',3,5),substr('hwgs HUAWEI ',3,5),substring('hwgs HUAWEI ',3,5);
       
       
 substrb |  substr  | substring
       
       
---------+----------+-----------
       
       
 gs Hua     | gs Huawei  | gs Huawei 
       
       
(1 row)
      
      
      
      

       
       
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2. Interception rules

GaussDB(DWS) Currently, three compatibility modes are supported ：ORA、TD and MySQL, Compatible with the function behavior of friends respectively , Improve the user migration experience . In different compatibility modes , The function difference is expressed as ：

substrb(string, s[, n])： The behavior of each compatibility mode is consistent

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       postgres=# select substrb('hwgs HUAWEI ',5,3),substrb('hwgs HUAWEI ',8,3);
       
       
 substrb | substrb
       
       
---------+---------
       
       
  Hua       |  by 
       
       
(1 row)
       
       
postgres=# select substrb('hwgs HUAWEI ',-6,3),substrb('hwgs HUAWEI ',-3,3);
       
       
 substrb | substrb
       
       
---------+---------
       
       
  Male       |  department 
       
       
(1 row)
       
       
postgres=# select substrb('hwgs HUAWEI ',5,0),substrb('hwgs HUAWEI ',8,-1);
       
       
 substrb | substrb
       
       
---------+---------
       
       
         |
       
       
(1 row)
      
      
      
      

       
       
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substr(string, s[, n])：s=0 There are compatible behavior differences

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       postgres=# select substr('hwgs HUAWEI ',5,3),substr('hwgs HUAWEI ',8,3);
       
       
 substr | substr
       
       
--------+--------
       
       
  Huawei  |  department 
       
       
(1 row)
       
       
postgres=# select substr('hwgs HUAWEI ',0,3),substr('hwgs HUAWEI ',0,3);
       
       
 substr | substr
       
       
--------+--------
       
       
 hwg    | hwg
       
       
(1 row)
       
       
mysql_db=# select substr('hwgs HUAWEI ',0,3),substr('hwgs HUAWEI ',0,3);
       
       
 substr | substr
       
       
--------+--------
       
       
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(1 row)
      
      
      
      

       
       
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substring(string, s[, n])：s<=0 and n<0 There are compatible behavior differences

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       postgres=# select substring('hwgs HUAWEI ',0,3),substring('hwgs HUAWEI ',-1,3);
       
       
 substring | substring
       
       
-----------+-----------
       
       
 hw        | h
       
       
(1 row)
       
       
td_db=# select substring('hwgs HUAWEI ',0,3),substring('hwgs HUAWEI ',-1,3);
       
       
 substring | substring
       
       
-----------+-----------
       
       
 hw        | h
       
       
(1 row)
       
       

       
       
mysql_db=# select substring('hwgs HUAWEI ',0,3),substring('hwgs HUAWEI ',-1,3);
       
       
 substring | substring
       
       
-----------+-----------
       
       
           |  department 
       
       
(1 row)
       
       
td_db=# select substring('hwgs HUAWEI ',0,-1);
       
       
ERROR:  negative substring length not allowed
       
       
CONTEXT:  referenced column: substring
       
       
mysql_db=# select substring('hwgs HUAWEI ',0,-1);
       
       
 substring
       
       
-----------
       
       

       
       
(1 row)
      
      
      
      

       
       
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Summary

Sum up , Introduce and summarize in detail substrb()、substr()、substring() Differences and usages of , In daily use , If the intercepted string is a multi byte character , Or the interception parameter may be a special value , Then you should pay special attention ; This article , I hope I can help you ！

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