PostgreSQL how to return data from anonymous code block? I have a table mytable (id, "someLongURI", status, "userId") and a working query: 

UPDATE          mytable
SET             status = 'IN_WORK'
WHERE           "someLongURI" IN (
    SELECT      "someLongURI"
    FROM        mytable
    WHERE       status = 'UNUSED'
    AND         pg_try_advisory_xact_lock(id)
    ORDER BY    id ASC 
    LIMIT       1
    FOR UPDATE 
)
RETURNING       id, "someLongURI";


But now I need to check the "userId" parameter and based on this either select existing row or update (and receive updated row). Something like that (in MySQL this would work):

IF NOT EXISTS (
    SELECT          1
    FROM            mytable tbl
    WHERE           tbl."userId" = 123
)
THEN
    UPDATE          mytable tbl
    SET             tbl.status = 'IN_WORK',
                    tbl."userId" = 123
    WHERE           tbl."someLongURI" IN (
        SELECT      "someLongURI"
        FROM        tbl.mytable
        WHERE       tbl.status = 'UNUSED'
            AND     pg_try_advisory_xact_lock(id)
        ORDER BY    id ASC 
        LIMIT       1
        FOR UPDATE 
    )
    RETURNING   tbl.id, tbl."someLongURI";
ELSE
    SELECT          tbl.id, tbl."someLongURI"
    FROM            mytable tbl
    WHERE           tbl."userId" = 123;
END IF;


I've wrapped it with DO $$ BEGIN {...} END $$; but it didn't work for     me. Last thing I came to: 

DO
    RETURNS TABLE (id INT4, "someLongURI" TEXT) AS
$$
DECLARE
    "_userId" INT4;
BEGIN
    "_userId" = 123;

    IF NOT EXISTS (
        SELECT          1
        FROM            mytable tbl
        WHERE           tbl."userId" = "_userId"
    )
    THEN
        UPDATE          mytable tbl
        SET             tbl.status = 'IN_WORK'
                        tbl."userId" = "_userId"
        WHERE           tbl."someLongURI" IN (
            SELECT      "someLongURI"
            FROM        tbl.mytable
            WHERE       tbl.status = 'UNUSED'
                AND     pg_try_advisory_xact_lock(id)
            ORDER BY    id ASC 
            LIMIT       1
            FOR UPDATE 
        )
        RETURNING       tbl.id, tbl."someLongURI";
    ELSE
        SELECT          tbl.id, tbl."someLongURI"
        FROM            mytable tbl
        WHERE           tbl."userId" = "_userId";
    END IF;
END $$;


Can't understand where I need to add "RETURNS" or some other statements inside this anonymous code block to receive the results of query
With Postgres you could do that in a single data modifying CTE without the need for an IF statement: 

WITH data AS (
    SELECT          tbl.id, tbl."someLongURI"
    FROM            mytable tbl
    WHERE           tbl."userId" = 123;
), changed AS (
    UPDATE          mytable tbl
    SET             tbl.status = 'IN_WORK',
                    tbl."userId" = 123
    WHERE           tbl."someLongURI" IN (
        SELECT      "someLongURI"
        FROM        tbl.mytable
        WHERE       tbl.status = 'UNUSED'
            AND     pg_try_advisory_xact_lock(id)
        ORDER BY    id ASC 
        LIMIT       1
        FOR UPDATE 
    )
    AND NOT EXISTS (SELECT * FROM data)
    RETURNING   tbl.id, tbl."someLongURI"
)
SELECT *
FROM changed
UNION ALL
SELECT *
FROM data
WHERE NOT EXISTS (SELECT * FROM changed);