Mysql: How do I write select where ids are not in another table

I have a simple structure of 2 tables:  contacts and group_contacts.

A 开发者_C百科contact can belong to one, many or no groups.

I'm trying to write a select statement that will give me all the contacts
that don't belong to group_id '123'. The negative, don't, has me confused.

CREATE TABLE IF NOT EXISTS `contacts` (
  `contact_id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
  `name` varchar(50) NOT NULL)

CREATE TABLE IF NOT EXISTS `group_contacts` (
  `contact_id` bigint(20) unsigned NOT NULL,
  `group_id` int(11) unsigned NOT NULL)



thanks

select a.contact_id from contacts a, group_contacts b where b.group_id<>123 and b.contact_id=a.contact_id;

You would have to proceed this in two macro-steps:

1. First you would need to left outer join between contacts and group_contacts table so that all contacts that do and don't have any relevant relation in group_contacts table is selected
2. Then in the where clause of group_contacts group_id of 123 should be loaded.

select * from contacts as ct 
left join group_contacts as gc on ct.contact_id=gc.contact_id
where gc.group_id!=123

try this

select * from contacts a left join  group_contacts b
on a.contact_id = b.contact_id
where  b.group_id !=123