如何联接两个表并从第一个表返回不在第二个表中的值？

Question

I have two tables, the first containing the id and employee_name, and the second containing the employee_id (same as id) and some other columns. How do I join the first table with the second so that I can output all the employee ID's (and the rest of the columns) in the first table that don't appear in the second? The query I've gotten to is this:

select employee_id
from data2 left join data1 on data2.employee_id = data1.id
where employee_id is NULL

This outputs the correct number of rows, but there are no values in them. Any help is greatly appreciated!

Answer 1

Your tables are flipped in your SQL. Based on your narrative, the SQL should be:

select data1.*
from data1
left join data2 on data2.employee_id = data1.id
where data2.employee_id is NULL

Answer 2

You need not use join, you can simply use 'not in' clause

select id
from table1
where id not in (select employee_id from table2)

Let me know, if it works.

Answer 3

If I understand your question correctly, you want to get the records from data1 whose values of employee_id do not exist in the column id in data2 .

The way the current query is structured, you are asking for employee_id ... where employee_id is NULL so that will only return null records. Instead, make a query of data1 like this:

select * from data1 where employee_id not in (select id from data2)

This uses the subquery to find a list of id values in data2 and then only returns records from data1 whose employee_id does not exist in that list.