数据库CLOB为空的判断方法是什么

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下列的哪些选项在执行之后会显示 "CLOB has a length of 0" ?

(A)

DECLARE   l_clob   CLOB;BEGIN   IF l_clob IS NULL   THEN      DBMS_OUTPUT.put_line ('CLOB is NULL');   ELSE      DBMS_OUTPUT.put_line ('CLOB has a length of '                             || LENGTH (l_clob));   END IF;END;/

SQL> DECLARE  2     l_clob   CLOB;  3  BEGIN  4     IF l_clob IS NULL  5     THEN  6        DBMS_OUTPUT.put_line ('CLOB is NULL');  7     ELSE  8        DBMS_OUTPUT.put_line ('CLOB has a length of '  9                               || LENGTH (l_clob)); 10     END IF; 11  END; 12  /CLOB is NULLPL/SQL procedure successfully completedSQL>

(B)

DECLARE   l_clob   CLOB := EMPTY_CLOB ();BEGIN   IF l_clob IS NULL   THEN      DBMS_OUTPUT.put_line ('CLOB is NULL');   ELSE      DBMS_OUTPUT.put_line ('CLOB has a length of '                             || LENGTH (l_clob));   END IF;END;/

SQL> DECLARE  2     l_clob   CLOB := EMPTY_CLOB ();  3  BEGIN  4     IF l_clob IS NULL  5     THEN  6        DBMS_OUTPUT.put_line ('CLOB is NULL');  7     ELSE  8        DBMS_OUTPUT.put_line ('CLOB has a length of '  9                               || LENGTH (l_clob)); 10     END IF; 11  END; 12  /CLOB has a length of 0PL/SQL procedure successfully completedSQL>

(C)

DECLARE   l_clob   CLOB := '';BEGIN   IF l_clob IS NULL   THEN      DBMS_OUTPUT.put_line ('CLOB is NULL');   ELSE      DBMS_OUTPUT.put_line ('CLOB has a length of '                             || LENGTH (l_clob));   END IF;END;/

SQL> DECLARE  2     l_clob   CLOB := '';  3  BEGIN  4     IF l_clob IS NULL  5     THEN  6        DBMS_OUTPUT.put_line ('CLOB is NULL');  7     ELSE  8        DBMS_OUTPUT.put_line ('CLOB has a length of '  9                               || LENGTH (l_clob)); 10     END IF; 11  END; 12  /CLOB is NULLPL/SQL procedure successfully completedSQL>

(D)

DECLARE   l_clob1   CLOB := EMPTY_CLOB ();   l_clob2   CLOB := l_clob1;BEGIN   IF l_clob2 IS NULL   THEN      DBMS_OUTPUT.put_line ('CLOB is NULL');   ELSE      DBMS_OUTPUT.put_line ('CLOB has a length of '                             || LENGTH (l_clob2));   END IF;END;/

SQL> DECLARE  2     l_clob1   CLOB := EMPTY_CLOB ();  3     l_clob2   CLOB := l_clob1;  4  BEGIN  5     IF l_clob2 IS NULL  6     THEN  7        DBMS_OUTPUT.put_line ('CLOB is NULL');  8     ELSE  9        DBMS_OUTPUT.put_line ('CLOB has a length of ' 10                               || LENGTH (l_clob2)); 11     END IF; 12  END; 13  /CLOB has a length of 0PL/SQL procedure successfully completed

答案BD. 注意答案C里面的 '' 相当于NULL.

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