当你像这样更改更新时会发生什么。

UPDATE CUSTOMERS
SET    NAME = 'MARIO',
       LASTNAME = 'PEREZ'
WHERE  customer_id IN (SELECT customer_id
                       FROM   CUSTOMERS
                       WHERE  CITY = 'SAN JOSE'
                       FETCH FIRST 100 ROWS ONLY)

注意：将表中的customer_id列改为identity或Auto generated列

就我所知，您的更新查询是有效的（您没有说明您使用的是哪个版本的db2，所以我假设是LUW 9.7或更高版本）。可能是squirrel向查询添加了一些东西，尝试取消选中限制squirrel接收行数的复选框。
如果你有唯一标识一行的东西（我假设这里是cust_id），你可以使用merge语句：

merge into CUSTOMERS x 
using ( 
    SELECT * FROM CUSTOMERS 
    WHERE CITY = 'SAN JOSE' 
    FETCH FIRST 100 ROWS ONLY 
) y 
    on x.cust_id = y.cust_id 
when matched then update             
    SET NAME = 'MARIO', LASTNAME = 'PEREZ';

另一个选项是枚举表中的行，如下所示：

UPDATE ( 
    SELECT x.*, row_number() over () as rn 
    FROM CUSTOMERS x 
    WHERE CITY = 'SAN JOSE' 
)    
    SET NAME = 'MARIO', LASTNAME = 'PEREZ' 
where rn <= 100;

如果表没有id，可以使用相对记录号。

UPDATE CUSTOMERS AS UPD SET SET NAME = 'MARIO', LASTNAME = 'PEREZ'
WHERE RRN(UPD)  IN 
(SELECT RRN(C)
    FROM CUSTOMERS AS C 
    WHERE CITY = 'SAN JOSE'
    FETCH FIRST 100 ROWS ONLY)