MySQL：选择两个表之间不通用的数据

Question

I have table1 with 3 numbers and table2 with 1 number:

table1       table2
n              n
1              1
2
3

I want to select data from table1 that is NOT present in table2 (numbes 2 and 3). I tried:

select table1.* from table1, table2 where table1.n <> table2.n

I also tried other where clauses:

where table1.n not like table2.n
where not table1.n = table2.n

But I don't get the results. I know it can be done in multiple steps, but I wonder if there is a simple query to do it. Thanks

Answer 1

You need to do a LEFT JOIN and look for null values for t2. Like this:

SELECT
   t1.n
FROM
   table1 AS t1
   LEFT JOIN table2 AS t2
     ON t1.n = t2.n
WHERE
    t2.n IS NULL

Here is link to a great reference for different sorts of JOINS which includes Venn diagrams to help you visualize the different approaches to joining.

http://blog.codinghorror.com/a-visual-explanation-of-sql-joins/

Answer 2

You can do that using NOT IN or NOT EXISTS.

select * from table1
where table1.n not in (select table2.n from table2);

select * from table1
where  not exists (select table2.n from table2 where table2.n = table1.n);

Answer 3

I tried both approaches (left join and not in) several times and it took the exact same time (my table is pretty big). Thanks guys!