OCP 1Z0 051 163

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163. View the Exhibit and examine the description for the CUSTOMERS table.
You want to update the CUST_CREDIT_LIMIT column to NULL for all the customers, where
CUST_INCOME_LEVEL has NULL in the CUSTOMERS table. Which SQL statement will accomplish the
task?

A. UPDATE customers
SET cust_credit_limit = NULL
WHERE CUST_INCOME_LEVEL = NULL;
B. UPDATE customers
SET cust_credit_limit = NULL
WHERE cust_income_level IS NULL;
C. UPDATE customers
SET cust_credit_limit = TO_NUMBER(NULL)
WHERE cust_income_level = TO_NUMBER(NULL);
D. UPDATE customers
SET cust_credit_limit = TO_NUMBER(' ',9999)
WHERE cust_income_level IS NULL;

现oracle中空字符串的值是NULL，但空格不是。D中的就是空格。
NULL 值不能用等号大于小于比较，只能用 IS NULL、IS NOT NULL。

TEST >select count(*),count(comm) from emp;  COUNT(*) COUNT(COMM)---------- -----------        14           41 row selected.TEST >select * from emp where comm > null;no rows selectedTEST >select * from emp where comm < null;no rows selectedElapsed: 00:00:00.01TEST >select * from emp where comm = null;no rows selectedTEST >select empno,comm from emp where comm is not null;     EMPNO       COMM---------- ----------      7499        300      7521        500      7654       1400      7844          04 rows selected.

to_number(null)结果仍然是NULL

TEST >set null NULLTEST >select to_number(null) from dual;TO_NUMBER(NULL)---------------NULL1 row selected.

Answer: B

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