防止count（）在mysql中返回“NULL”

Question

在我的 mysql 查询中，我正在计算元素的数量（如果存在）。如果产品存在则返回实际计数，但如果没有产品则返回NULL。如果值为 null，我如何返回 0？我发现了与此问题相关的One、Two 线程。但它们都没有帮助。

我的查询是

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

我也试过IFNULL(count(*), 0)。它也给NULL

请帮助我...任何帮助将不胜感激。提前谢谢你

Answer 1

你应该封装它：

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