使用同一个表中的子查询进行SQL更新和增量列

Question

这是my fiddle。我的问题：

update foos
set foo_id = ((SELECT max_foo_id FROM (SELECT MAX(foo_id) AS max_foo_id FROM foos) AS temp_foos) + 1)
where foo_id is null; 

为foo_ids生成值1, 2, 2, 2（子查询只运行一次？）但我希望它能给我1, 2, 3, 4

Answer 1

怎么样？

UPDATE  foos a
        INNER JOIN
        (
            SELECT  a.bar_ID,
                    @rn := @rn + 1 row_Num
            FROM    foos a,(SELECT @rn := (SELECT MAX(foo_ID) FROM foos)) b
            WHERE   a.foo_ID IS NULL
        ) b ON a.bar_ID = b.bar_ID
SET     a.foo_id = b.row_Num

• SQLFiddle Demo

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更新1

• SQLFiddle Demo (with ORDER BY clause)