WHERE中的SQL子查询返回有限的结果集，但仍与所有结果匹配

Question

I need help with SQL query:

UPDATE content_type_blog b
LEFT JOIN node n
ON n.nid = b.nid
SET b.field_is_latest_value = 1
WHERE n.nid IN (
  (
    SELECT nid
    FROM node
    GROUP BY uid
    ORDER BY created DESC
  )
);

The select in WHERE clause returns 4012 results but when the whole query is run it updates 124k results instead limiting it to those 4012 matched in subquery.

This query intends to update content_type_blog - table columns by nid where created is highest (latest) in node-table.

Answer 1

Your subquery filters out duplicate nid values, but it still returns all of them. So it's not a filter at all.

In other databases, what you ask is typically done with a window function like row_number() . But MySQL doesn't support that. Instead, you could use a filtering join.

Here's an example. SQL Fiddle seems down, so I can't test it, but I hope it will point you in the right direction.

create table table1 (col1 int, col2 bit, created datetime)
insert table1 values
    (1, 0, '2013-01-01'),
    (1, 0, '2013-01-02'),
    (1, 0, '2013-01-03'),
    (2, 0, '2013-01-01');

update  table1
join    (
        select  col1
        ,       max(created) as max_created
        from    table1
        group by
                col1
        ) filter
on      filter.col1 = table1.col1
        and filter.max_created = table1.creatd
set     col2 = 1

select  *
from    table1

Answer 2

Maybe the problem would be that you are using content_type_blog b LEFT JOIN , so doing it you are including all rows of content_typ_blog on the result query. Try using INNER JOIN to update only the rows of your subquery:

UPDATE content_type_blog b
INNER JOIN node n
ON n.nid = b.nid
SET b.field_is_latest_value = 1
WHERE n.nid IN (
  (
    SELECT nid
    FROM node
    GROUP BY uid
    ORDER BY created DESC
    LIMIT 1
  )
);

Answer 3

This seems to do the trick:

UPDATE content_type_blog b
LEFT OUTER JOIN node n
ON n.nid = b.nid
JOIN    (
        SELECT DISTINCT nid,
        MAX(created) AS max_created
        FROM    node
        GROUP BY uid
        ) filter
ON      filter.nid = b.nid
SET b.field_is_latest_value = 1;