OCP-1Z0-051 第3题 转义字符

一、原题

You need to extract details of those products in the SALES table where the PROD_ID column contains the string '_D123'.
Which WHERE clause could be used in the SELECT statement to get the required output?
A. WHERE prod_id LIKE '%_D123%' ESCAPE '_'
B. WHERE prod_id LIKE '%\_D123%' ESCAPE '\'
C. WHERE prod_id LIKE '%_D123%' ESCAPE '%_'
D. WHERE prod_id LIKE '%\_D123%' ESCAPE '\_'

         答案 B

二、题目翻译

     要从SALES表中提取出PROD_ID列包含'_D123'字符串的产品明细。

     下面哪一个WHERE条件子句可以获得所需的输出结果？

三、题目解析

      ESCAPE子句为指定转译字符，因为'_'下画线在LIKE子句中指的是任意一个字符，所以需要把'_'下画线进行转义，转义字符设置为'\'。

四、实验

SQL> drop table test;

Table dropped.

SQL> create table test(id int,prod_id varchar2(20));

Table created.

SQL> insert into test values(1,'123_D123abc');

1 row created.

SQL> insert into test values(2,'123abc');

1 row created.

SQL> insert into test values(3,'D123');

1 row created.

SQL> insert into test values(4,'Da123');

1 row created.

SQL> commit;

Commit complete.

SQL> select * from test;

        ID PROD_ID

---------- ----------------------------------------

         1 123_D123abc

         2 123abc

         3 D123

         4 Da123

SQL> select * from test

  2  WHERE prod_id LIKE '%_D123%' ESCAPE '_';

WHERE prod_id LIKE '%_D123%' ESCAPE '_'

                   *

ERROR at line 2:

ORA-01424: missing or illegal character following the escape character

SQL> select * from test

  2  WHERE prod_id LIKE '%\_D123%' ESCAPE '\';

        ID PROD_ID

---------- ----------------------------------------

         1 123_D123abc

SQL> select * from test

  2  WHERE prod_id LIKE '%_D123%' ESCAPE '%_';

WHERE prod_id LIKE '%_D123%' ESCAPE '%_'

                                    *

ERROR at line 2:

ORA-01425: escape character must be character string of length 1

SQL> select * from test

  2  WHERE prod_id LIKE '%\_D123%' ESCAPE '\_';

WHERE prod_id LIKE '%\_D123%' ESCAPE '\_'

                                     *

ERROR at line 2:

ORA-01425: escape character must be character string of length 1