Oracle SQL 单引号与双引号区别

1.3 SQL 单引号与双引号

单引号:

• 表示字符串常量，并不区分大小写。。
• 字符串中的双引号仅仅被看成一个普通字符进行处理。此时，双引号不须要成对出现。
• 转义符，对紧随其后出现的字符(单引号)进行转义 ' '' ' ----> '
• 用于，字符串数据的操做，如建立表，查询，删除，添加，数据等

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

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

SQL> select '' from dual;

'
-

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


#转移字符单引号
SQL> select '''' from dual;

'
-
'

SQL> select ' '' ' from dual;

'''
---
 '

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

''''
----
 ''

---

双引号：

• 假如创建对象的时候，对象名、字段名加双引号，则示意 Oracle将严格区分大小写，不然Oracl都默认大写。
• 用于表名，字段名， 别名
• 出如今to_char的格式字符串中时，双引号有特殊的做用，就是将非法的格式符包装起来
• 用于建立用户时的密码，DB_link, 多用于数据库的操做

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;  --数字

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中能够运用其它分隔符（$,<>,[],{}等）,留心，运用这些分隔符时，不只要在分隔符先后加单引号，还要带有前缀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

'
#假如创建对象的时候，对象名、字段名加双引号，则示意 Oracle将严格区分大小写，不然Oracl都默认大写

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