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前期準備，新建資料表
create table mytable (
id serial not null,
"name" character varying(20),
geom geometry,
remark character varying(255) )
插入資料
insert into mytable(name,geom,remark) values('test1','point(12 23)','remark1');
insert into mytable(name,geom,remark) values('test2','point(14 23)','remark2');
insert into mytable(name,geom,remark) values('test3','point(17 23)','remark3');
insert into mytable(name,geom,remark) values('test4','point(92 29)','remark4');
insert into mytable(name,geom,remark) values('test5','point(82 83)','remark5');
1、 字串連線符 '||'
例子：select name || remark as newname from mytable where name like '%test%';
結果：newname
"test1remark1"
"test2remark2"
"test3remark3"
"test4remark4"
"test5remark5"
2、 字串長度：bit長度bit_length(string),字元長度char_length(string)或者是character_length(string),位元組長度octet(string)，具體的長度跟資料庫字元編碼有關，注意中文字元的長度
例子：select name,bit_length(name),char_length(name),octet_length(name) from mytable where name like '%test%';
結果"test1";40;5;5
"test2";40;5;5
"test3";40;5;5
"test4";40;5;5
"test5";40;5;5
3、 字串替換overlay(string placing string from int [for int])
例子：select name,overlay(name placing 'dd' from 5 for 2) from mytable where name like '%test%';
結果"test1";"testdd"
"test2";"testdd"
"test3";"testdd"
"test4";"testdd"
"test5";"testdd"
4、 查詢字串位子position(substring in string)
例子：select name,position('test' in name) from mytable where name like '%test%';
結果："test1";1
"test2";1
"test3";1
"test4";1
"test5";1
5、 取子串，substring(string [from int] [for int]),substring(string from pattern)
例子1：select name,substring(name from 1 for 2) from mytable where name like '%test%';
結果1：
"test1";"te"
"test2";"te"
"test3";"te"
"test4";"te"
"test5";"te"
例子2：select name,substring(name from '^...') from mytable where name like '%test%';
結果2：以任何三個字元開頭的，以任何兩個字元結尾的為：substring(name from '..$')
"test1";"tes"
"test2";"tes"
"test3";"tes"
"test4";"tes"
"test5";"tes"
6、 轉換為大些和小寫：upper(string)和lower(string)
例子：select name,upper(name),lower(name) from mytable where name like '%test%';
結果："test1";"test1";"test1"
"test2";"test2";"test2"
"test3";"test3";"test3"
"test4";"test4";"test4"
"test5";"test5";"test5"
7、 去除開頭或者結尾或者全部的包含的字串trim([leading|trailing|both] [characters] from string)
例子：select name,trim(name),trim(leading 'test' from name),trim('1' from name) from mytable where name like '%test%';
結果："test1";"test1";"1";"test"
"test2";"test2";"2";"test2"
"test3";"test3";"3";"test3"
"test4";"test4";"4";"test4"
"test5";"test5";"5";"test5"
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