左外连接条件 sql

Question

would anybody know if there is way to do conditional left outer join? I have ledger tables with actuals and reference department table from which I want to assign BMDIV. But department reference table contains LC number or = 'xx' What I want to do is to join ledger table with department table on LCTRYNUM, LC and DEPTNUM but in the case of LC = '**' join would be only LCTRYNUM and DEPTNUM.

Ledger acutal table

LCTRYNUM	LC	DEPTNUM	Amount
618	40	30813	100
618	50	30813	200
618	60	30813	300
618	10	30813	100

Business Division reference table

LCTRYNUM	LC	DEPTNUM	BMDIV
618	**	30813	30
618	10	30813	2P

Expected result would be this

LCTRYNUM	LC	DEPTNUM	Amount	BMDIV
618	40	30813	100	30
618	50	30813	200	30
618	60	30813	300	30
618	10	30813	100	2P

Answer 1

You can use two left join s, the first to bring in direct matches and the second for the default match:

select a.*, coalesce(d.bmdiv, d_default.bmdiv) as bmdiv
from actual a left join
     divisions d
     on a.lc = d.lc left join
     divisions d_default
     on d_default.lc = '**'