在when子句中替换MySQL查询中的空值

Question

I am having a MySQL Table as One Below

CREATE TABLE Test(Name VARCHAR(30));

INSERT INTO Test VALUES ('Name1'), 
                        ('Name2'),
                        (null),
                        ('Name3'),
                        (null),
                        ('Name4');

Now when the row is Null I want it to be Displayed as Empty in front end else the value contained in the row

I tried the following query but it ain't worked

SELECT CASE Name 
        WHEN NULL 
        THEN '' ELSE Name
       END AS Names
  FROM Test

Output

Name1
Name2

Name3

Name4

The link for SQLFiddle

Thanks for Reply

Answer 1

Null values are special in SQL, you can't treat them as ordinary expressions. Although you can assign NULL to a column, you can't compare with it as if it's a normal value; all comparison functions return NULL if either of the arguments is NULL , and this is treated as false (so NULL = NULL and 1 <> NULL are both false). You're required to use special expressions to compare with NULL . The most basic form of this is <expression> IS NULL (or <expression> IS NOT NULL ), but there are also convenience functions IFNULL() and COALESCE() .

SELECT IFNULL(Name, '') AS Names
FROM Test

or

SELECT CASE WHEN Name IS NULL
            THEN ''
            ELSE Name
       END AS Names
FROM Test

For more information, see 3.3.4.6. Working with NULL Values in the MySQL documentation.

Answer 2

Just to elaborate on why this doesn't work.

A NULL is a value for which comparison yields neither a true nor false result. That's one of the oddities of the three-tiered logic intrinsic to database programming.

NULL = NULL  

is not true. It's not false either. The outcome of any comparison of one value to a NULL value is a NULL result, unless you use the intrinsic SQL " IS NULL " operator to test it.

In the form of CASE statement you were using:

CASE Value
   WHEN TestValue1 THEN xxx
   WHEN TestValue2 THEN yyy
END

a number of equality tests are run against a value. You cannot test for NULL in this form, because you cannot do equality testing for NULL. The second form of case:

CASE
   WHEN MyValue IS NULL THEN www
   WHEN Condition1 THEN xxx
   WHEN Condition2 THEN yyy
END

does allow you to use an IS NULL expression, which can have a TRUE or FALSE value. Thus, when you want to do NULL testing in a CASE statement, you must always use this form.

Also, the NULL test should be the first item in the CASE statement: because comparing any value against NULL will have an "undetermined" (NULL) boolean value, and the logic of the case will not work properly otherwise.