sql - I need to get NULL values in Columns if Intermediate table doesn't apply -

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i joining tables there 2 separate relationship schemes so:

tablea1    ->    tablea2      ->  tableb     ->  tablec    : <<< relationship enter code here

and output needs so:

   a1columnid   a2columnid  bcolumnid   ccolumnid    1            1           1           1    2            null        2           2    3            null        3           3    4            2           4           4    5            null        5           5

if there exists these 2 relationship & b , need see a2colid null below relationship:

and 2nd relationship scheme looks so: tablea1->tableb-tablec <<<< relationship b (this scheme has no clue tablea2)

how join result set nulls in a2columnid depicted above?

left join instead of regular join force table join if no data exists in right table, filling in null in place. when join a1 , a2 so:

select *    a1         left join a2            on a1.idlink = a2.idlink         join b            on a1.id = b.id         join c            on a1.id = c.id;

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