mysql - Insert missing records from table -

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i have 2 tables users (id, username, etc...) , user_contacts (id, userid, contactid, etc...).

given have user w/ id of 84, efficient query in order have missing records inserted user_contacts associate user 84 w/ other users?

given recent comment, should work:

insert user_contacts (userid, contactid) select u.id, 84 users u     left join user_contacts uc on u.id = uc.userid , uc.contactid = 84 uc.id null

this insert row user_contacts table each user don't have row contactid 84. sure specify columns correctly.

alternatively, use not in or not exists. example,

insert user_contacts (userid, contactid) select u.id, 84 users u u.id not in (     select userid      user_contacts      contactid = 84)

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