Use of not exists? 1

I have two tables(A,B), one of them initially empty
I want to do a select from A if B is empty or if B.field1 does not have a specific value or is NULL.
So something like

SELECT A.field1, A.field2 FROM A JOIN B ON A.field1=B.field1 WHERE [B is empty]
OR B.field2 !=A.field2

How can one do that in mysql?

 

[jpadie]

there are a number of methods to achieve what I think you are trying to achieve. the best is a trade off between various mysql query techniques (left outer joins, not exist statements, not in statements, is null etc) dependent on the table size, structure, indices etc.

try this (without sample create and insert statements it is difficult to test)

SELECT  a.field1, a.field2
FROM    tableA a
WHERE   a.field1 NOT IN
        (
        SELECT  b.field1
        FROM    tableB c
        )
OR      0 = 
        (
        SELECT COUNT(*)
        FROM    tableB c
        WHERE   c.field1 = a.field1 
        AND     c.field2 = a.field2
        )