如果表c中的列id是主键，那么它应该没有问题，但是如果不是，请尝试添加另一个条件来过滤子查询结果，例如，
例如，以下是开始日期：

SELECT a.column_1, b.column_2, 
        (SELECT column_3 FROM tableC  
        WHERE (id = a.id 
        AND (start_date = (SELECT MAX(b.start_date) 
             from tableC as c
             where a.id = c.id ))) AS column_3
FROM tableA as a INNER JOIN
     tableB as b ON b.id = a.id
WHERE b.id_user = 50;

你在混合两种东西。中的标量子查询 SELECT 列表应该只返回一个值（行和列）。使用 LIMIT 1 将得到一行，但仍有许多列。因此可以指定列名：

select a.*, b.*,
  (select c.col_name from tableC c where c.id_tableA = a.id order by .. limit 1) 
from tableA a 
inner join tableB b on a.id = b.id_tableA 
where b.id_user = 50;

或者用普通的 JOIN :

select a.*, b.*, c.*
from tableA a
inner join tableB b 
  on a.id = b.id_tableA 
left join tableC c
  on c.id_tableA = a.id
where b.id_user = 50;