First , Understand a prerequisite ：
where 1 = 1; -- Always true
in other words ：
select * from student;
Equate to ：
select * from student where 1 = 1;
look ,where 1 = 1 Is a condition that is always true , It's no use . But when we add dynamics SQL After the statement , The result is different ：
No, where 1 = 1 when
select * from student
where
and condition1
and condition2
and ...
When all condition All for false when , The statement becomes ：
select * from student where;
This SQL The statement is grammatically wrong .
Yes where 1 = 1 when
select * from student
where 1 = 1
and condition1
and condition2
and ...
When all condition All for false when , The statement becomes ：
select * from student where 1 = 1;
because where The condition after the statement is true, Therefore, this statement is logically equivalent to
select * from student;
summary
where 1 = 1 Statement for dynamic SQL In the sentence , It is a method to construct a correct and running dynamic query in order to meet the uncertain factors in the multi condition query page SQL A method of statement .
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