一个选项是在以下范围内使用LEFT JOIN：

• 每一位客户与每年
• 您的查询

然后对缺失的Sales应用COALESCE函数。

WITH combinations AS (
    SELECT clients.Client, years.Year
    FROM       (SELECT DISTINCT Client FROM MySalesTable) clients
    INNER JOIN (SELECT DISTINCT Year FROM MySalesTable) years ON 1=1
) 
SELECT Client,
       Year,
       COALESCE(total_sales, 0) AS total_sales
FROM      combinations
LEFT JOIN (SELECT Client, Year, SUM(Sales) AS total_sales
           FROM MySalesTable 
           GROUP BY Client, Year) sales
       ON combinations.Client = sales.Client 
      AND combinations.Year = sales.Year
ORDER BY Client, Year DESC

最明显的解决方案（尽管效率很低）是从同一个表中获取一个由年份组成的列表。

SELECT a.client, allyears.year, sum(a.sales) 
FROM mysalestable
FROM (SELECT DISTINCT year FROM MySalesTable) allyears 
LEFT JOIN MySalesTable a
ON allyears.year=a.year
....

然而，这仍然不会列出一年中没有销售额的客户--实际上，您需要一个数据源来生成所有年份和所有客户的列表--这需要一个笛卡尔连接......

SELECT allclients.client, allyears.year, sum(a.sales) 
FROM (SELECT DISTINCT year FROM MySalesTable) allyears
JOIN (SELECT DISTINCT client FROM MySalesTable) allclients 
LEFT JOIN MySalesTable a
ON allyears.year=a.year
AND allclients.client=a.client
GROUP BY allclients.client, allyears.year
ORDER BY allclients.client, allyears.year DESC

(You可能有其他表，查询所有年份和所有客户端的成本较低）
不幸的是，MySQL没有实现row generator的机制。

我想这对你会有帮助：
How to SELECT based on value of another SELECT

select t1.*, t2.* from (
SELECT Client, Year, SUM(Sales) FROM MySalesTable GROUP BY Client, Year
) as t1,
(
SELECT Client, Year, SUM(Sales) FROM MySalesTable GROUP BY Client, Year
) as t2
where t1.Client = t2.Client

甚至可以内部连接表