sql - 如果没有找到值，如何在 MySQL 中使用 SUM 函数返回“0”

Question

我希望我的 sum 函数返回 '0'，如果值不存在，任何人都可以确定以下 SQL 命令是否有问题。我已经改变了合并的顺序，但这并没有什么不同

SqlCommand scGetPostings = new SqlCommand(@"
SELECT 
  D1.dr, 
  D1.cr, 
  D1.asset_no, 
  (open_bal + dr - cr) AS closing_balance 
FROM (SELECT 
        COALESCE(SUM(dr_amount), 0) AS dr, 
        COALESCE(SUM(cr_amount), 0) AS cr, 
        asset_no 
      FROM posting, sysasset 
      WHERE posting.asset_no = @AssetNumber 
        AND period >= asset_open_per 
      GROUP BY asset_no) AS D1, asset 
WHERE D1.asset_no = asset.asset_no", DataAccess.AssetConnection);

Answer 1

if condition在您的声明中使用：

SqlCommand scGetPostings = new SqlCommand(@"
SELECT 
  D1.dr, 
  D1.cr, 
  D1.asset_no, 
  (open_bal + dr - cr) AS closing_balance 
FROM (SELECT 
        SUM(if(dr_amount is null , 0 , dr_amount )) AS dr, 
        SUM(if(cr_amount is null , 0 , cr_amount )) AS cr, 
        asset_no 
      FROM posting, sysasset 
      WHERE posting.asset_no = @AssetNumber 
        AND period >= asset_open_per 
      GROUP BY asset_no) AS D1, asset 
WHERE D1.asset_no = asset.asset_no", DataAccess.AssetConnection);

Answer 2

尝试这个：

SELECT 
  asset.asset_no, 
  COALESCE(dr, 0) AS dr, 
  COALESCE(cr, 0) AS cr, 
  (open_bal + COALESCE(dr, 0) - COALESCE(cr, 0)) AS closing_balance 
FROM 
    asset LEFT JOIN
     (SELECT 
        SUM(dr_amount) AS dr, 
        SUM(cr_amount) AS cr, 
        asset_no 
      FROM posting, sysasset 
      WHERE posting.asset_no = @AssetNumber 
        AND period >= asset_open_per 
      GROUP BY asset_no) AS D1 
ON D1.asset_no = asset.asset_no