Bulk Insert / Update / Delete

I want to be able to generate SQL statements that will go through a list of
data, effectively row by row, enquire on the database if this exists in the
selected table- If it exists, then the colums must be UPDATED, if not, they
must be INSERTED.

Logically then, I would like to SELECT * FROM <TABLE>
WHERE ....<Values entered here>, and then IF FOUND
UPDATE <TABLE> SET .... <Values entered here> ELSE
INSERT INTO <TABLE> VALUES <Values entered here>
END IF;

The IF statement gets rejected by the parser. So it would appear that
PostgreSQL does not support an IF in this type of query, or maybe not at
all.

Does anyone have any suggestions as to how I can achieve this ?

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