SQL Statement 1=1

It's supposed to increase performance on a SQL query that otherwsie doesn't have any conditions. It also allows you to build dynamic queries because with the WHERE 1=1 at the end of your query, all you have to add is AND clauses for your actual conditions.

Other then JSOP said the nasty SQL injection is also using the same.

Suppose there's one query

SELECT * FROM Table WHERE userid='"+TextBox1.Text+"';

at a time of passing textbox value as hiren' OR '1'='1

will make Query looking like this

SELECT * FROM Table WHERE userid='hiren' OR '1'='1'

for getting list of all user information using SQL Injection.

See some more example of Injection HERE[^], Please use it for good cause for powering security in your query rather then applying Injection for deflating others.

Could be just a placeholder for real clause, like "if (true) {/* ... */}"