Table used to add restrictions for learning spells from trainer. Player can't learn spell 'spell_id' till he learns 'req_spell', when he loses 'req_spell' 'spell_id' will be lost too. Table is used also for profession specialisations as specialisations require profession of certain rank to learn.
表结构
Spell ID from Spell.dbc, which require knowing `req_spell` to learn from trainer.
Spell ID from Spell.dbc, which is required to be known before `spell_id` can be learned from trainer.