大约有 31,500 项符合查询结果（耗时：0.0256秒） [XML]
Difference between CTE and SubQuery?
... most useful for recursion:
WITH hier(cnt) AS (
SELECT 1
UNION ALL
SELECT cnt + 1
FROM hier
WHERE cnt < @n
)
SELECT cnt
FROM hier
will return @n rows (up to 101). Useful for calendars, dummy rowsets etc.
They are also more readable (i...
How does HashSet compare elements for equality?
...ploye() { Name = "Rob" });
Display(hashSetB);
var union = hashSet.Union<Employe>(hashSetB).ToList();
Display(union);
var inter = hashSet.Intersect<Employe>(hashSetB).ToList();
Display(inter);
var except = hashSet.Ex...
Best way to select random rows PostgreSQL
...ner
) r
JOIN big b USING (id) -- eliminate miss
UNION -- eliminate dupe
SELECT b.*
FROM (
SELECT 1 + trunc(random() * 5100000)::int AS id
FROM random_pick r -- plus 3 percent - adapt to your needs
LIMIT ...
What is the most efficient/elegant way to parse a flat table into a tree?
...RECURSIVE MyTree AS (
SELECT * FROM MyTable WHERE ParentId IS NULL
UNION ALL
SELECT m.* FROM MyTABLE AS m JOIN MyTree AS t ON m.ParentId = t.Id
)
SELECT * FROM MyTree;
I tested recursive queries in MySQL 8.0 in my presentation Recursive Query Throwdown in 2017.
Below is my original an...
Is String.Format as efficient as StringBuilder
...var referToTheComputedValuesSoCompilerCantOptimiseTheLoopsAway= concatkeys.Union(stringbuilderkeys).Union(cachedsbkeys).Union(formatkeys).LastOrDefault(x=>x[1]=='-');
Console.WriteLine(referToTheComputedValuesSoCompilerCantOptimiseTheLoopsAway);
...
What guarantees are there on the run-time complexity (Big-O) of LINQ methods?
...).
Distinct, GroupBy Join, and I believe also the set-aggregation methods (Union, Intersect and Except) use hashing, so they should be close to O(N) instead of O(N²).
Contains checks for an ICollection implementation, so it may be O(1) if the underlying collection is also O(1), such as a HashSet<...
Split function equivalent in T-SQL?
...1,12,13,14,15'
SET @delimiter = ','
;WITH cte AS
(
SELECT 0 a, 1 b
UNION ALL
SELECT b, CHARINDEX(@delimiter, @str, b) + LEN(@delimiter)
FROM CTE
WHERE b > a
)
SELECT SUBSTRING(@str, a,
CASE WHEN b > LEN(@delimiter)
THEN b - a - LEN(@delimiter)
ELSE LEN(@str) - a +...
How to output MySQL query results in CSV format?
...u can include header by prepending "SELECT 'order_id','product_name','qty' UNION" before the real query. First select query returns header, second query returns real data; UNION joins it together.
– petrkotek
Jun 14 '13 at 4:35
...
how to concatenate two dictionaries to create a new one in Python? [duplicate]
...ems() for d in (d1, d2, d3)))
and:
from itertools import chain
def dict_union(*args):
return dict(chain.from_iterable(d.items() for d in args))
Python 2.6 & 2.7
from itertools import chain
dict(chain.from_iterable(d.iteritems() for d in (d1, d2, d3))
Output:
>>> from iterto...
Build an ASCII chart of the most commonly used words in a given text [closed]
...umn FROM OPENROWSET(BULK'A',
SINGLE_BLOB)x;WITH N AS(SELECT 1 i,LEFT(@,1)L UNION ALL SELECT i+1,SUBSTRING
(@,i+1,1)FROM N WHERE i<LEN(@))SELECT i,L,i-RANK()OVER(ORDER BY i)R INTO #D
FROM N WHERE L LIKE'[A-Z]'OPTION(MAXRECURSION 0)SELECT TOP 22 W,-COUNT(*)C
INTO # FROM(SELECT DISTINCT R,(SELECT''+...