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 ：

 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

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 .

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)

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

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)

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

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
--------+--------
        |
(1 row)

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

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)

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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