大约有 40,000 项符合查询结果（耗时：0.0205秒） [XML]
Selecting the last value of a column
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Another good resource is the selected answer at productforums.google.com/forum/#!topic/docs/p3t3feg7Jic which shows how to get the first, last, or nth row in a FILTER()ed range, similar to @Geta's answer.
– Aaron Blenkush
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Count number of records returned by group by
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You can do both in one query using the OVER clause on another COUNT
select
count(*) RecordsPerGroup,
COUNT(*) OVER () AS TotalRecords
from temptable
group by column_1, column_2, column_3, column_4
share
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django : using select_related and get_object_or_404 together
Is there any way of using get_object_or_404 and select_related together or any other way to achieve the result of using these two together(except from putting it in try/except)??
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SQL Query to concatenate column values from multiple rows in Oracle
...on on string aggregation techniques. A very common one is to use LISTAGG:
SELECT pid, LISTAGG(Desc, ' ') WITHIN GROUP (ORDER BY seq) AS description
FROM B GROUP BY pid;
Then join to A to pick out the pids you want.
Note: Out of the box, LISTAGG only works correctly with VARCHAR2 columns.
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How to check if a column exists in a SQL Server table?
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SQL Server 2005 onwards:
IF EXISTS(SELECT 1 FROM sys.columns
WHERE Name = N'columnName'
AND Object_ID = Object_ID(N'schemaName.tableName'))
BEGIN
-- Column Exists
END
Martin Smith's version is shorter:
IF COL_LENGTH('schemaName.tabl...
Generate MD5 hash string with T-SQL
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Use HashBytes
SELECT HashBytes('MD5', 'email@dot.com')
That will give you 0xF53BD08920E5D25809DF2563EF9C52B6
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SELECT CONVERT(NVARCHAR(32),HashBytes('MD5', 'email@dot.com'),2)
That will give you F53BD08920E5D25809DF2563EF9C52B6
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NOT IN vs NOT EXISTS
...llow NULLs the NOT IN will be treated identically to the following query.
SELECT ProductID,
ProductName
FROM Products p
WHERE NOT EXISTS (SELECT *
FROM [Order Details] od
WHERE p.ProductId = od.ProductId)
The exact plan may vary but for my examp...
Is there a way to access the “previous row” value in a SELECT statement?
...d to order by some column for this to be meaningful. Something like this:
select t1.value - t2.value from table t1, table t2
where t1.primaryKey = t2.primaryKey - 1
If you know how to order things but not how to get the previous value given the current one (EG, you want to order alphabetically) ...
Not equal != operator on NULL
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As a manual workaround, you could commonly SELECT * FROM MyTable WHERE coalesce(MyColumn, 'x') <> 'x' to assign a constant if it is NULL value, providing you give an appropriate datatype for the sentinel value x (in this case a string/char). This is TSQL syntax ...
Is there any performance gain in indexing a boolean field?
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It depends on the actual queries and the selectivity of the index/query combination.
Case A: condition WHERE isok = 1 and nothing else there:
SELECT *
FROM tableX
WHERE isok = 1
If the index is selective enough (say you have 1M rows and only 1k have isok = 1), ...