如何从另一个表中不存在列的表中进行选择

Question

table A

id    city  
1    koronadal  
2    cebu  
3    manila  

Table B

id    city   
1    cebu  

Expected ouput:

id    city
1    koronadal  
3    manila  

Here is my query:

 Select a.id, a.city from tablea a left join tableb b on a.city = b.city

I get the wrong output.. please help me..

Answer 1

You are on the right way! now you just need to filter those who match :

Select a.id, a.city
from tablea a
left join tableb b 
 on a.city = b.city
WHERE b.id is null

In LEFT JOINING when there is no match of the join condition, the left table, in this case tableA will still be selected, and all the data from the right table, in this case tableB will be NULL . So all you need to do is search for null values on B table.

Another approach would be to use IN() :

SELECT * FROM TableA a
WHERE a.city NOT IN(SELECT city FROM TableB)

And another way is EXISTS() :

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