WHERE NOT EXISTS in PostgreSQL gives syntax error

Question

When trying to use the WHERE NOT EXISTS clause to prevent adding a row with a duplicate value in the column age, I get the error syntax error at or near "WHERE".

Why did it throw a syntax error? I'm using Postgresql 9.1.

SQL

INSERT INTO live.users ("website", "age") 
values ('abc', '123')
WHERE NOT EXISTS (SELECT age FROM live.users WHERE age = 123);

Error

ERROR:  syntax error at or near "WHERE"
LINE 6: WHERE NOT EXISTS (SELECT age FROM live.users W...

Answer 1

Do instead:

INSERT INTO live.users ("website", "age") 
SELECT 'abc', 123
WHERE NOT EXISTS (SELECT age FROM live.users WHERE age = 123);

Answer 2

INSERT INTO live.users ("website", "age") 
select 'abc', '123'
WHERE NOT EXISTS (SELECT age FROM live.users WHERE age = 123);

Answer 3

I encountered some issues in using WHERE field NOT EXISTS in PLPGSQL. Instead what worked well was WHERE field NOT IN, I received no function errors after using that.

Answer 4

I see you asked for v9.1 but it's been 4 yrs since and now, starting from PostgreSQL v9.5 - INSERT gives you ON CONFLICT … DO NOTHING option:

INSERT INTO live.users("website", "age") VALUES('abc', '123') ON CONFLICT ("age") DO NOTHING

Worth noting this requires respective constraint set up on the target table - but in most cases, I imagine you would have it anyway. Otherwise you'll get:

ERROR:  there is no unique or exclusion constraint matching the ON CONFLICT specification