Using Union All and Order By in MySQL

Question

I've 2 tables:

create table advertised_products(id int,title varchar(99),timestamp timestamp);
insert advertised_products select 1,'t1',curdate();

create table wanted_products(id int,title varchar(99),timestamp timestamp);
insert wanted_products select 1,'t1',now();

I'm using this query to get the records:

(
SELECT 
    ap.*,
    'advertised'  as type 
FROM advertised_products as ap
)
union all
(
SELECT 
    wp.*,
    'wanted' as type 
FROM wanted_products as wp
)
ORDER BY timestamp desc limit 3

But it gives error:

Column 'timestamp' in order clause is ambiguous

How can i sort this?

Answer 1

Wrap it in a subquery.

SELECT s.*
FROM
    (
        SELECT  ap.*, 'advertised'  as type 
        FROM advertised_products as ap
          union all
        SELECT  wp.*, 'wanted' as type 
        FROM wanted_products as wp
    ) s
ORDER BY s.timestamp desc 
limit 3