mysql - SQL join not working (or very slow) -

i have following tables in mysql:

table a:

+-------+-------------+------+-----+---------+-------+ | field | type        | null | key | default | | +-------+-------------+------+-----+---------+-------+ | sid   | varchar(50) | yes  |     | null    |       | | type  | int(11)     | yes  |     | null    |       | +-------+-------------+------+-----+---------+-------+ 

table b:

+---------+-------------+------+-----+---------+-------+ | field   | type        | null | key | default | | +---------+-------------+------+-----+---------+-------+ | channel | varchar(20) | yes  |     | null    |       | | sid     | varchar(50) | yes  |     | null    |       | | type    | varchar(20) | yes  |     | null    |       | +---------+-------------+------+-----+---------+-------+ 

i want find rows have entry in b same sid. tried following join command:

select a.sid join b on a.sid=b.sid; 

this query never gives me answer. tabe has 465420 entries , table b has 291326 entries.

why not work? there many entries? or have fact have no primary keys assigned?

your query fine. appear need index. suggest b(sid).

you can write query as:

select a.sid exists (select 1 b a.sid = b.sid); 

this not affect performance -- unless there lots of duplicates in b -- eliminate issues caused duplicates in b.