What is 'where 1=1' and 'where null=null' in sql server

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I have a table of some columns named as Students.

I am using following three queries to select rows.

1.

SQL

select * from Students

2.

SQL

select * from Students where 1=1

3.

SQL

select * from Students where null=null

When I execute first then this returns all rows.

When I execute second then this is also returns all rows.

But when I execute third then this does not return any row.

I have two issues

What is 1=1 and null=null and why are we using these?
What is difference between first query and second query?

Posted 6-Jun-15 1:34am

manojmaks

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2 solutions

Solution 2

This would work though. When comparing NULL you should always use "IS"

SQL

where null is null

Posted 10-Jul-15 0:07am

JR1965

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OriginalGriff · Accepted Answer · 2015-06-06T01:51:00

SQL

SELECT ... WHERE 1=1

is a pointless waste: 1=1 evaluates to true, so all records will be returned. It's functionally identical to the same select without the where clause (but harder to read, and probably slightly less efficient as the command has to be parsed)

SQL

SELECT ... WHERE null=null

is different: nulls propagate in SQL because they aren't "real" values - just a marker for a non-existent value - so any expression with a null in it, returns null. Since null is not true, the WHERE clause always fails, and no rows are returned.

See here: http://en.wikipedia.org/wiki/Null_(SQL)[^]