在多行的情況下使用WITH子句進行執行

Question

我試圖將PERFORM與WITH查詢一起使用，該查詢返回多行。

CREATE OR REPLACE FUNCTION test_function() RETURNS void AS $$ 

BEGIN 

PERFORM (
WITH selection AS (
    SELECT id, 
    ROW_NUMBER() OVER w AS r, 
    first_value(id) OVER w AS first_value, 
    nth_value(id, 5) OVER w AS last_value 
    FROM mytable 
    WINDOW w AS (PARTITION BY v.ability_id ORDER BY unit_id ASC) 
) 
create_question(id, 1, 1, 1) 
FROM selection 
WHERE ability_id IN (
    SELECT ability_id 
    FROM selection 
    WHERE last_value > 0.5 
    ORDER BY first_value DESC 
) 
AND selection.r <= 5 
); 

END; 
$$ LANGUAGE plpgsql; 

，我得到的錯誤：

ERROR: more than one row returned by a subquery used as an expression 

Postgres的醫生說不能這樣做：

For WITH queries, use PERFORM and then place the query in parentheses. (In this case, the query can only return one row.)

可以做些什麼，以書面形式，從解決這個問題，除了With查詢（在這裏稱爲選擇）兩次？

Answer 1

備註：您的查詢不到create_question(id, 1, 1, 1)之前SELECT。

訣竅是修改查詢，以便它返回一行。

您可以通過使用聚合函數（例如，寫：

SELECT 
    count(create_question(id, 1, 1, 1)) 
FROM selection 
... 

然後查詢只返回單行，並且可以用作PERFORM語句子查詢。