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

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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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