属性值的空白值

Question

Suppose I have

 SELECT A,B FROM T1 WHERE A=X and B=Y;

The way I set it up along with php, I know there is a possibility that X and Y may be empty, since I am giving the choice to search with A and/or B. So let's say only A is given, what would Y have to be to return all tuples where A=X? Is there some kind of 'blank' function for SQL?

Answer 1

The following will return all rows where either or A or B is NULL (never filled in) or both A and B equal to something you defined:

SELECT A, B FROM T1 WHERE (A IS NULL or A=X) and (B IS NULL or B=Y);

If you were looking for something else, please clarify your question.