Duplicate NULL values

Question

Hi Experts,
It's a challenging question to all experts
Q1)
If we create an UNIQUE Key on a table Why it's accepting morethan one NULL value.
Q2)
If we use DISTINCT key word on a column that contains morethan one NULL value why it's displaying one NULL value.
Q3)
If we use UNION on a column that contains morethan one NULL value, why it returns one NULL value.
Q4)
If we write a query like this
SELECT * FROM emp WHERE comm IS NULL
Why it returns all the employes those contain no comm.
For 1st question some pleople are answering any NULL is not equal to other NULL value so that it accepting multiple NULL values.
For 2nd,3rd question some people are answering because DISTINCT and UNION eliminates duplicate values.
According to the first question answer how it is possible.
For 4th question answer some people are saying all the NULLs are equal so that it displaying all the employess which are having no commision.
Any One Plese help how NULL works in all four scenarios.
Thanks in advance.

Answer 1

user9077483 wrote:
Hi Experts,
It's a challenging question to all experts
Q1)
If we create an UNIQUE Key on a table Why it's accepting morethan one NULL value.It depends really. Are you talking a SINGLE COLUMN unique index? If so, Oracle won't index the NULLs values, hence you can put in as many as you want.
If it's a concatenated INDEX then the number of NULL values may be limited. See below for an example:
SQL> CREATE TABLE TEST(TXT VARCHAR2(10), TXT2 VARCHAR2(10));
Table created.
SQL> CREATE UNIQUE INDEX TEST_TXT_UNQ ON TEST(TXT);
Index created.
SQL> INSERT INTO TEST(TXT) VALUES(NULL);
1 row created.
SQL> INSERT INTO TEST(TXT) VALUES(NULL);
1 row created.
SQL> CREATE UNIQUE INDEX TEST_TXT_UNQ2 ON TEST(TXT,TXT2);
Index created.
SQL> INSERT INTO TEST(TXT,TXT2) VALUES('A',NULL);
1 row created.
SQL> INSERT INTO TEST(TXT,TXT2) VALUES('A',NULL);
INSERT INTO TEST(TXT,TXT2) VALUES('A',NULL)
ERROR at line 1:
ORA-00001: unique constraint (TUSER.TEST_TXT_UNQ) violated
Q2)
If we use DISTINCT key word on a column that contains morethan one NULL value why it's displaying one NULL value.Per the definition of DISTINCT:
Specify DISTINCT or UNIQUE if you want the database to return only one copy of each set of duplicate rows selected. These two keywords are synonymous. Duplicate rows are those with matching values for each expression in the select list.It returns only one copy of a ROW even if a column in a row contains NULLs.
>
Q3)
If we use UNION on a column that contains morethan one NULL value, why it returns one NULL value.For the same reason as above UNION removes duplicates. Use UNION ALL if you want duplicate values.
>
Q4)
If we write a query like this
SELECT * FROM emp WHERE comm IS NULL
Why it returns all the employes those contain no comm.Because you asked it to? The IS NULL condition is special in that it checks the column to see if it contains a NULL value. If it is the condition evaluates to true. You can't use WHERE COMM = NULL because you can't use an equality operator to compare NULL values.