错误：“SELECT”处或附近的语法错误

Question

I am really new to postgres. The question looks very simple but I just cant see where I got wrong.

I a table created as follows:

  CREATE TABLE IF NOT EXISTS t(
        tn VARCHAR(30) NOT NULL,
        PRIMARY KEY(tn)
    );

I want to insert an instance if the instance does not exist. Here is my code:

INSERT INTO t (tn) 
VALUES 
(SELECT 'q' WHERE NOT EXISTS (SELECT * FROM t WHERE tn = 'q')) ;

And the psql console keeps giving me the error

ERROR:  syntax error at or near "SELECT"

I have checked every piece of code individually, for instance both

SELECT 'q' WHERE NOT EXISTS (SELECT * FROM t WHERE tn = 'q');

and

INSERT INTO t (tn) VALUES ('p');

run without error. But error occurs when I put them together.

Does anyone know where I got wrong..?

Answer 1

Lose VALUES and the brackets...

INSERT INTO t (tn) 
SELECT 'q' WHERE NOT EXISTS (SELECT * FROM t WHERE tn = 'q');