如何从A表中选择B表中不存在的值？

Question

I have 2 tables in mySQL DB: A & B.

a, b, c columns.

Table A:

 a(1) = 1 
 a(2) = 2
 a(3) = 3

Table B:

 a(1) = 1 
 a(2) = 2

So, we could see that in B table there is no row with a = 3. How could I request DB to find it?

So response (one row) could looks like:

a(1) = 3 
b(1) =..
c(1) =.. 

Answer 1

One option uses EXISTS :

SELECT a.a
FROM TableA a
WHERE NOT EXISTS (SELECT 1 FROM TableB b WHERE b.a = a.a);

Another option would be to do an anti-join:

SELECT a.a
FROM TableA a
LEFT JOIN TableB b
    ON a.a = b.a
WHERE b.a IS NULL;

Answer 2

You can also give up joins and use WHERE and nested SELECT: suppose TabA holds values of 1,2,3 in subsequent rows of column ValA and TabB holds values of 1,2 in subsequent rows of column ValB and you want only a row containing value of 3 from TabA you can do this without joins:

SELECT  Val_A
FROM TabA  
WHERE Val_A NOT IN (SELECT Val_B FROM TabB)