Predict the number of tuples in SQL result

Question

Consider the following relation instance myTable

num1	num2
100	100
100	NULL
NULL	100
NULL	NULL

 

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Query 1:

SELECT *

FROM myTable

WHERE num1 != NULL;

Query 2:

SELECT *

FROM myTable

WHERE num1 = NULL;

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If Query 1 returns m tuples and Query 2 returns n tuples, then 10m+n will be  _____________

Comments

query 2 should throw error as we cant use simply null , we have keyword for it , i.e isnull.

SELECT * FROM myTable
WHERE Address IS NULL

The above is correct query.

Edit: 

How can we test for NULL values?

It is not possible to test for NULL values with comparison operators, such as =, <, or <>. so probbly your question is wrong.

References: http://www.w3schools.com/sql/sql_null_values.asp

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Tried this query in mySQL:

create table gate2017 (id int, id2 int);
INSERT INTO gate2017 VALUES (null,null);
select * from gate2 where id=null;

Working fine..

Please refer this also: https://en.wikipedia.org/wiki/Null_(SQL)#Controversy

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Answer is 20 ?.

Answer 1

num1 != null and num1 = null are always false . So no rows will be selected by executing both the queries.

10*m+ n =0

Answer 2

SQL allows queries that check whether an attribute value is NULL. Rather than using

= or <> to compare an attribute value to NULL, SQL uses the comparison operators IS or

IS NOT. This is because SQL considers each NULL value as being distinct from every

other NULL value, so equality comparison is not appropriate. It follows that when a join

condition is specified, tuples with NULL values for the join attributes are not included in

the result (unless it is an OUTER JOIN).

-From Navathe (7th edition, chapter 7).

Answer 3

NULL is not a value, and therefore cannot be compared to another value.

where x is null checks whether x is a null value.

where x = null is checking whether x equals NULL, which will never be true

Comments

what about query 1 x!=null

Can m=4 ??