sql - Table Join condition -

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i have 2 tables   table       :                   item    lookup                          x                   b       null                   c        y                   d        k  table b       :                       lookup                      x                      y                      z  want join these tables , output                 item    lookup                      x                b       null                c       y

i wanna pick matching lookup , null lookups in output view. can tell me join condition

it looks want rows in a either match b or have null lookup. inner join , special condition:

select distinct a.item, a.lookup tablea join      tableb b      on (a.lookup = b.lookup) or (a.lookup null);

if have index on lookup, exists better performance:

select a.item, a.lookup tablea a.lookup null or       exists (select 1 tableb b b.lookup = a.lookup);

edit:

using left join where condition possible:

select distinct a.item, a.lookup tablea left join      tableb b      on a.lookup = b.lookup (a.lookup null , b.lookup null) or       b.lookup not null;

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