SQL查询语法错误，使用带有2列的select＆<>

Question

I got a table with 2 columns here.

| column1 | column2 |
| A       | 1       |
| B       | 1       |
| C       | 2       |
| D       | 1       |

I'm trying to execute a SELECT query that: Selects a value FROM Column 2 WHILE Column 2 doesn't have the same value in a row which has Column 1 = "A"

SELECT column2 from mytable and column2 <> (SELECT * FROM mytable where column1 = 'A');

Basically I'm trying to execute a query that returns column1 values only when column2 is valued at "2" here. But the project I'm making will be having column2 values random so I should use only columns names

Sorry if that's too confusing!

Answer 1

use corelated subquery

  select t1.* from mytable t1
  where not exists ( select 1 from mytable t2 where t2.column2=t1.column2 and column1='A')

or use not in

select t1.* from table_name t1 where t1.column2 not in ( select column2 from table_name where column1='A')

Answer 2

The subquery of your query returns multiple values. So you have to use NOT IN instead of <> :

SELECT column2 
FROM mytable 
WHERE column2 NOT IN (SELECT column2 FROM mytable WHERE column1 = 'A');

Answer 3

You could also use a join , for example:

select t1.column2
from mytable t1 left join 
(select distinct t2.column2 from mytable t2 where t2.column1='A') t3 on t1.column2=t3.column2
where t3.column2 is null