大约有 6,888 项符合查询结果（耗时：0.0265秒） [XML]
Using LIMIT within GROUP BY to get N results per group?
...
For me something like
SUBSTRING_INDEX(group_concat(col_name order by desired_col_order_name), ',', N)
works perfectly. No complicated query.
for example: get top 1 for each group
SELECT
*
FROM
yourtable
WHERE
id IN (SELECT
S...
SQL - Update multiple records in one query
... VALUES ";
$len = count($array);
foreach ($array as $index => $values) {
$sql .= '("';
$sql .= implode('", "', $array[$index]) . "\"";
$sql .= ')';
$sql .= ($index == $len - 1) ? "" : ", \n";
}
$sql .= "\nON DU...
First-time database design: am I overengineering? [closed]
...you will take will largely be dependent on having and optimizing the right indexes for your particular queries / procedures and more importantly the volume of records. Unless you are talking about well over a million records in your main tables you seem to be on track to having a sufficiently mains...
Database development mistakes made by application developers [closed]
...vely easy one but still it happens all the time. Foreign keys should have indexes on them. If you're using a field in a WHERE you should (probably) have an index on it. Such indexes should often cover multiple columns based on the queries you need to execute.
2. Not enforcing referential integri...
Get absolute path of initially run script
... If I'm using a front controller pattern. Would __FILE__ return the index.php's location or the included file's location? How would I get the index.php's location?
– CMCDragonkai
Nov 15 '13 at 23:55
...
How do I prevent Eclipse from hanging on startup?
...SPACE_DIR/.metadata/.plugins/org.eclipse.core.resources/.projects \
-name .indexes -exec rm -fr {} \;
That did the trick.
Initially I thought it was a problem with Mylyn (I experienced these freezes after I started using its generic web connector), but the problem appeared even after uninstalling...
Counting null and non-null values in a single query
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SQLServer uses an index scan for this query vs two index seeks using a union. On a table with 40.000 rows, there is no speed difference.
– Lieven Keersmaekers
Aug 13 '09 at 13:45
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In SQL, what's the difference between count(column) and count(*)?
... The answer posted by @Alan states "count(*) is computed by looking at the indexes on the table in question rather than the actual data rows" which, if true, invalidates your comment. I appreciate that @Alan may be wrong but I'm interested in the source of your information in order to find out which...
How to delete all data from solr and hbase
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If you want to clean up Solr index -
you can fire http url -
http://host:port/solr/[core name]/update?stream.body=<delete><query>*:*</query></delete>&commit=true
(replace [core name] with the name of the core you want t...
Extract elements of list at odd positions
...efault (0) would be used. So the first element (at position 0, because the indexes are 0-based) would be selected. In this case the second element will be selected.
Because the second element is omitted, the default is being used (the end of the list). So the list is being iterated from the second ...