你的代码正在执行 CROSS JOIN 两个查询的结果。
因此，如果其中任何一个没有返回行，则结果是没有行。
如果您确定这两个查询只返回一行，请将它们的结果用作列：

SELECT 
  (SELECT Quantity FROM Table1 WHERE IdProduct = 1234) qty,
  (SELECT Supplier FROM Table2 WHERE IdProduct = 1234) supplier

对我来说，像这样把这些问题拼凑在一起是很了不起的。但是，如果您想让它在其中一个表中缺少记录的情况下生存，则需要一个 OUTER JOIN 某种程度上。看到了吗https://dev.mysql.com/doc/refman/5.7/en/join.html.

你可能想要一个“完全的外部连接”。不幸的是，mysql没有实现完全的外部连接（为什么在这个时代？）。
无论如何，您可以使用解决方法来模拟它：

SELECT
   bloc1.Quantity AS qty,
   bloc2.Supplier AS supplier
FROM (SELECT Quantity FROM Table1 WHERE IdProduct = 1234) bloc1
LEFT JOIN (SELECT Supplier FROM Table2 WHERE IdProduct = 1234) bloc2 on 1 = 1
UNION
SELECT
   bloc1.Quantity AS qty,
   bloc2.Supplier AS supplier
FROM (SELECT Quantity FROM Table1 WHERE IdProduct = 1234) bloc1
RIGHT JOIN (SELECT Supplier FROM Table2 WHERE IdProduct = 1234) bloc2 on 1 = 1

在mysql 8.x中，您可以通过使用ctes删除冗余（并稍微缩短它）。例如：

with
bloc1 as (SELECT Quantity FROM Table1 WHERE IdProduct = 1234),
bloc2 as (SELECT Supplier FROM Table2 WHERE IdProduct = 1234)
SELECT
   bloc1.Quantity AS qty,
   bloc2.Supplier AS supplier
FROM bloc1 LEFT JOIN bloc2 on 1 = 1
UNION
SELECT
   bloc1.Quantity AS qty,
   bloc2.Supplier AS supplier
FROM bloc1 RIGHT JOIN bloc2 on 1 = 1

你可以试试下面的-

select 
(SELECT Quantity FROM Table1 WHERE IdProduct = 1234) as qty,
(SELECT Supplier FROM Table2 WHERE IdProduct = 1234) as supplier

你可以连接两个表table1和table2你可以做：

SELECT qty,sup
    FROM   (SELECT Quantity as qty,supplier as sup
        FROM Table1 
        INNER JOIN Table2
        ON table1.IdProduct=table2.IdProduct AND IdProduct=1234);

你可以查看这个来获得更多关于mysql中连接的信息https://www.mysqltutorial.org/mysql-join/