sql - Conditional Self Joins -


suppose have data in table :

trade id | source -----------------  x        | lch       x        | commodore   y        | lch           z        | commodore

i output :

x | lch            y | lch            z | commodore

meaning if there "lch" entry given trade id, take precedence.

i have done in java or other programming language getting basic results after joining tables , filter using map etc. due performance reasons using query @ first place.

could please have , let me know if there solution in sql.

this task windowed aggregate function, no need join:

select trade_id, source  (    select trade_id, source,       row_number()        on (partition trade_id             oder case when source = 'lch' 0 else 1 end) rn    your_table  ) dt rn = 1

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