New answers tagged mysql-5.5
1
(Addressing first question)
Toss comment_unseen; instead, use a LEFT JOIN ... WHERE ... IS NULL or WHERE NOT EXISTS ( SELECT 1 FROM comment_seen WHERE ... )
For comment_seen, replace
CONSTRAINT user_id UNIQUE (user_id, comment_id)
with this (Note: a PK includes a uniqueness constraint):
PRIMARY KEY(user_id, comment_id)
If you need to look up which users ...
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