What does this SQL statement "WHERE table_a.key (+)= table

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i reading through lecture notes of school, , come across slide:

chapter: data integration , etl process

slide title: duplicate values problem

text: duplicate values invariably exist. eliminating can time consuming, although simple task perform.

sql example:

select ... table_a, table_b table_a.key (+)= table_b.key union select ... table_a, table_b table_a.key = table_b.key (+);

specifically, not understand meaning of (+)= , last (+).

thanks helping!

it means should stop using old-style joins condition in where clause , always use explicit join syntax.

for particular query, equivalent full outer join:

select ... table_a full outer join      table_b      on table_a.key = table_b.key;

Yet