大约有 44,000 项符合查询结果（耗时：0.0289秒） [XML]
How do I check if an integer is even or odd? [closed]
...tring();
if (String.IsNullOrEmpty(foo))
return Evenness.Unknown;
char bar = foo[foo.Length - 1];
switch (bar)
{
case '0':
case '2':
case '4':
case '6':
case '8':
return Evenness.Even;
case '1':
case '3':
case '5':
case '7':
case...
Which is fastest? SELECT SQL_CALC_FOUND_ROWS FROM `table`, or SELECT COUNT(*)
...irm this - I just updated a query with 4 joins on a 168,000 row database. Selecting just the first 100 rows with a SQL_CALC_FOUND_ROWS took over 20 seconds; using a separate COUNT(*) query took under 5 seconds (for both count + results queries).
– Sam Dufel
Ju...
How do SQL EXISTS statements work?
... criteria -- this is why it can be faster than IN. Also be aware that the SELECT clause in an EXISTS is ignored - IE:
SELECT s.*
FROM SUPPLIERS s
WHERE EXISTS (SELECT 1/0
FROM ORDERS o
WHERE o.supplier_id = s.supplier_id)
...should hit a division by zero error...
How to see if an object is an array without using reflection?
...d have to check if the object is an instanceof boolean[], byte[], short[], char[], int[], long[], float[], double[], or Object[], if you want to detect all array types.
Also, an int[][] is an instanceof Object[], so depending on how you want to handle nested arrays, it can get complicated.
For the t...
COUNT(*) vs. COUNT(1) vs. COUNT(pk): which is better? [duplicate]
...for all your queries that need to count everything, even for joins, use *
SELECT boss.boss_id, COUNT(subordinate.*)
FROM boss
LEFT JOIN subordinate on subordinate.boss_id = boss.boss_id
GROUP BY boss.id
But don't use COUNT(*) for LEFT joins, as that will return 1 even if the subordinate table doe...
How to select a single field for all documents in a MongoDB collection?
...nt such as:
db.student.find({}, {roll:1, _id:0})
The above statement will select all documents in the students collection, and the returned document will return only the roll field (and exclude the _id).
If we don't mention _id:0 the fields returned will be roll and _id. The '_id' field is always d...
How to create a MySQL hierarchical recursive query
... or self-joins.
MySQL 8+
with recursive cte (id, name, parent_id) as (
select id,
name,
parent_id
from products
where parent_id = 19
union all
select p.id,
p.name,
p.parent_id
from products p
inner join cte
...
Select first row in each GROUP BY group?
As the title suggests, I'd like to select the first row of each set of rows grouped with a GROUP BY .
17 Answers
...
Check whether an input string contains a number in javascript
...pattern" is the regular expression itself, and "modifiers" are a series of characters indicating various options.
The character class is the most basic regex concept after a literal match. It makes one small sequence of characters match a larger set of characters. For example, [A-...
Define variable to use with IN operator (T-SQL)
... VALUES (2)
INSERT INTO @MyList VALUES (3)
INSERT INTO @MyList VALUES (4)
SELECT *
FROM MyTable
WHERE MyColumn IN (SELECT Value FROM @MyList)
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