Oracle SQL 單引號與雙引號區別

1.3 SQL 單引號與雙引號

單引號:

• 表示字符串常量，並不區分大小寫。。
• 字符串中的雙引號僅僅被看成一個普通字符進行處理。此時，雙引號不須要成對出現。
• 轉義符，對緊隨其後出現的字符(單引號)進行轉義 ' '' ' ----> '
• 用於，字符串數據的操做，如建立表，查詢，刪除，添加，數據等

SQL> select 'hh24"小時""mi"分"""ss"秒"' AS RESULT from dual;

RESULT
-----------------------------------------
hh24"小時""mi"分"""ss"秒"

SQL> select '' from dual;

'
-

SQL> select ''' from dual;
ERROR:
ORA-01756: quoted string not properly terminated


#轉移字符單引號
SQL> select '''' from dual;

'
-
'

SQL> select ' '' ' from dual;

'''
---
 '

SQL> select ' '''' ' from dual;

''''
----
 ''

---

雙引號：

• 假如創建對象的時候，對象名、字段名加雙引號，則示意 Oracle將嚴格區分大小寫，不然Oracl都默認大寫。
• 用於表名，字段名， 別名
• 出如今to_char的格式字符串中時，雙引號有特殊的做用，就是將非法的格式符包裝起來
• 用於建立用戶時的密碼，DB_link, 多用於數據庫的操做

SQL> select to_char(sysdate, 'hh24"H"mi"M"ss"S"') AS RESULT from dual;

RESULT
---------
07H08M37S

#建立User
CREATE USER CH1
  IDENTIFIED BY 'pass'
  DEFAULT TABLESPACE TBS_01
  TEMPORARY TABLESPACE TEMP
  PROFILE DEFAULT
  ACCOUNT UNLOCK;
  
Error at line 1
ORA-00988: missing or invalid password(s)

CREATE USER CH1
  IDENTIFIED BY "pass"
  DEFAULT TABLESPACE TBS_01
  TEMPORARY TABLESPACE TEMP
  PROFILE DEFAULT
  ACCOUNT UNLOCK;
  
#建立DB_Link
CREATE DATABASE LINK "ORCL_HR.QA.WEBEX.COM"
 CONNECT TO HR
 IDENTIFIED BY "pass"
 USING 'tony1521';
 
 SQL> select first_name from emp where "employee_id" = 200;
select first_name from emp where "employee_id" = 200
                                 *
ERROR at line 1:
ORA-00904: "employee_id": invalid identifier


SQL> select first_name from emp where "EMPLOYEE_ID" = 200;

FIRST_NAME
----------------------------------------
Jennifer

SQL> select first_name as "name" from emp where employee_id = 200;

name
----------------------------------------
Jennifer

SQL> select first_name  'name' from emp where employee_id = 200;
select first_name  'name' from emp where employee_id = 200
                   *
ERROR at line 1:
ORA-00923: FROM keyword not found where expected

---

注意

SQL> select first_name as "name" from emp where employee_id = 200;  --數字

name
----------------------------------------
Jennifer

SQL> select first_name as "name" from emp where employee_id = '200'; --字符

name
----------------------------------------
Jennifer

SQL> select first_name as "name" from emp where employee_id = "200"; 
select first_name as "name" from emp where employee_id = "200"
                                                         *
ERROR at line 1:
ORA-00904: "200": invalid identifier

#在oracle10g中能夠運用其它分隔符（$,<>,[],{}等）,留心，運用這些分隔符時，不只要在分隔符先後加單引號，還要帶有前綴q.
SQL> SELECT 'D''COSTA' name FROM DUAL;

NAME
-------
D'COSTA

SQL> SELECT q'$D'COSTA$' NAME FROM DUAL;

NAME
-------
D'COSTA

SQL> SELECT q'[D'COSTA]' NAME FROM DUAL;

NAME
-------
D'COSTA

SQL> SELECT q'<D'COSTA>' NAME FROM DUAL;

NAME
-------
D'COSTA

'
#假如創建對象的時候，對象名、字段名加雙引號，則示意 Oracle將嚴格區分大小寫，不然Oracl都默認大寫

SQL> create table test(a varchar2(10), a varchar2(10));
create table test(a varchar2(10), a varchar2(10))
                                  *
ERROR at line 1:
ORA-00957: duplicate column name

SQL> create table test("a" varchar2(10), a varchar2(10));

Table created.

SQL> desc test;
 Name                                      Null?    Type
 ----------------------------------------- -------- ----------------------------
 a                                                  VARCHAR2(10)
 A                                                  VARCHAR2(10)

SQL> create table test_1(a varchar2(1), "A" varchar2(1));
create table test_1(a varchar2(1), "A" varchar2(1))
                                   *
ERROR at line 1:
ORA-00957: duplicate column name