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当作一个普通字符进行处理。此时，双引号不需要成对出现。
• 转义符，对紧随其后出现的字符(单引号)进行转义 ' '' ' ----> '
• 用于，字符串数据的操作，如创建表，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：

• 假如建立对象的时候，对象名、字段名加双引号，则示意 Oracle将严格区分大小写，否则Oracl都默认大写。
• 用于表名，字段名， 别名
• 出现在to_char的格式字符串中时，双引号有特殊的作用，就是将非法的格式符包装起来
• 用于创建用户时的密码，DB_link, 多用于数据库的操作

SQL> select to_char(sysdate, 'hh24"H"mi"M"ss"S"') AS RESULT from dual;RESULT
---------
07H08M37S#创建User
CREATE USER CH1IDENTIFIED BY 'pass'DEFAULT TABLESPACE TBS_01TEMPORARY TABLESPACE TEMPPROFILE DEFAULTACCOUNT UNLOCK;Error at line 1
ORA-00988: missing or invalid password(s)CREATE USER CH1IDENTIFIED BY "pass"DEFAULT TABLESPACE TBS_01TEMPORARY TABLESPACE TEMPPROFILE DEFAULTACCOUNT UNLOCK;#创建DB_Link
CREATE DATABASE LINK "ORCL_HR.QA.WEBEX.COM"CONNECT TO HRIDENTIFIED 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 identifierSQL> select first_name from emp where "EMPLOYEE_ID" = 200;FIRST_NAME
----------------------------------------
JenniferSQL> select first_name as "name" from emp where employee_id = 200;name
----------------------------------------
JenniferSQL> 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
----------------------------------------
JenniferSQL> select first_name as "name" from emp where employee_id = '200'; --字符name
----------------------------------------
JenniferSQL> 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中可以运用其它分隔符（$,<>,[],{}等）,留心，运用这些分隔符时，不仅要在分隔符前后加单引号，还要带有前缀q.
SQL> SELECT 'D''COSTA' name FROM DUAL;NAME
-------
D'COSTASQL> SELECT q'$D'COSTA$' NAME FROM DUAL;NAME
-------
D'COSTASQL> SELECT q'[D'COSTA]' NAME FROM DUAL;NAME
-------
D'COSTASQL> SELECT q'<D'COSTA>' NAME FROM DUAL;NAME
-------
D'COSTA'
#假如建立对象的时候，对象名、字段名加双引号，则示意 Oracle将严格区分大小写，否则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 nameSQL> 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