防止計數（）在mysql中返回「NULL」

Question

在我的mysql查詢中，如果存在元素，我將計數它。如果產品存在，它會返回實際計數，但如果沒有產品，則返回NULL。如果值爲空，我怎麼能返回0？我發現One，Two與此問題有關的線索。但他們都沒有幫助。

我的查詢是

SELECT 
    COUNT(*) AS num 
FROM 
    table1 AS t1 
JOIN table2 AS t2 ON t2.id = t1.id_fk 
JOIN table3 AS t3 ON t3.id = t2.t3_id_fk 
JOIN table4 AS t4 ON t4.id = t2.t4_id_fk 
JOIN table 5 AS t5 ON t5.t2_id_fk = t2_id 
WHERE pd.product_url = 'sample_product' 
AND mcd.main_category_url = 'sample_category' 
AND scd.sub_category_url = 'sample_sub_category' 
AND mpd.merchant_prod_status = 'active' 
AND pd.product_status = 'active' 
AND mcd.main_category_status = 'active' 
AND scd.sub_category_status = 'active' 
GROUP BY t1.id 

我也試過IFNULL(count(*), 0)。它也給NULL

請幫助我...任何幫助將不勝感激。謝謝你在前進

Answer 1

你應該將其封裝：

SELECT IFNULL(num,0) 
    FROM (SELECT COUNT(*) AS num 
      FROM table1 AS t1 
      JOIN table2 AS t2 ON t2.id = t1.id_fk 
      JOIN table3 AS t3 ON t3.id = t2.t3_id_fk 
      JOIN table4 AS t4 ON t4.id = t2.t4_id_fk 
      JOIN table 5 AS t5 ON t5.t2_id_fk = t2_id 
      WHERE pd.product_url = 'sample_product' 
      AND mcd.main_category_url = 'sample_category' 
      AND scd.sub_category_url = 'sample_sub_category' 
      AND mpd.merchant_prod_status = 'active' 
      AND pd.product_status = 'active' 
      AND mcd.main_category_status = 'active' 
      AND scd.sub_category_status = 'active' 
     GROUP BY t1.id 
    ) s