MySQL update many-to-many tabel relation -

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i have tables , b , a_has_b have fields:

a: id, name

b: id, name

a_has_b: a_id, b_id, background

previously when had not 'background' field in a_has_b, did update delete record in a_has_b , reinsert updated records.

but how can update a_has_b whitout deleteing records should reinsert?

off top of head, think like....

delete a_has_b not exists (   select 1 b   b.id=a_has_b.b_id );

(and similar table 'a')

or

delete a_has_b b.id <> (select id b);

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