大约有 40,000 项符合查询结果（耗时：0.0274秒） [XML]
How do MySQL indexes work?
...be faster to go through the pages one by one (in a database, this is "poor selectivity").
For a 10-page book, it makes no sense to make an index, as you may end up with a 10-page book prefixed by a 5-page index, which is just silly - just scan the 10 pages and be done with it.
The index also needs t...
SQL Query Where Field DOES NOT Contain $x
...t is meant to be used in subqueries or with predefined lists:
-- subquery
SELECT a FROM x WHERE x.b NOT IN (SELECT b FROM y);
-- predefined list
SELECT a FROM x WHERE x.b NOT IN (1, 2, 3, 6);
If you are searching a string, go for the LIKE operator (but this will be slow):
-- Finds all rows where...
Postgresql GROUP_CONCAT equivalent?
...
This is probably a good starting point (version 8.4+ only):
SELECT id_field, array_agg(value_field1), array_agg(value_field2)
FROM data_table
GROUP BY id_field
array_agg returns an array, but you can CAST that to text and edit as needed (see clarifications, below).
Prior to version...
SQL Call Stored Procedure for each Row without using a cursor
...rate over all customers
WHILE (1 = 1)
BEGIN
-- Get next customerId
SELECT TOP 1 @CustomerID = CustomerID
FROM Sales.Customer
WHERE CustomerID > @CustomerId
ORDER BY CustomerID
-- Exit loop if no more customers
IF @@ROWCOUNT = 0 BREAK;
-- call your sproc
EXEC dbo.YOURSPROC...
Not equal != operator on NULL
...
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 ...
Overloaded method selection based on the parameter's real type
...
I expect the method selection to take
in consideration the real (not the
declared) parameter type. Am I missing
something?
Yes. Your expectation is wrong. In Java, dynamic method dispatch happens only for the object the method is called ...
Case in Select Statement
I have an SQL statement that has a CASE from SELECT and I just can't get it right. Can you guys show me an example of CASE where the cases are the conditions and the results are from the cases. For example:
...
Select objects based on value of variable in object using jq
...
Adapted from this post on Processing JSON with jq, you can use the select(bool) like this:
$ jq '.[] | select(.location=="Stockholm")' json
{
"location": "Stockholm",
"name": "Walt"
}
{
"location": "Stockholm",
"name": "Donald"
}
...
How do I execute a stored procedure once for each row returned by query?
...
declare @field1 int
declare @field2 int
declare cur CURSOR LOCAL for
select field1, field2 from sometable where someotherfield is null
open cur
fetch next from cur into @field1, @field2
while @@FETCH_STATUS = 0 BEGIN
--execute your sproc on each row
exec uspYourSproc @field1, @fiel...
Xcode duplicate/delete line
...ct with the following content and restart Xcode.
{
"^$K" = (
"selectLine:",
"cut:"
);
"^$D" = (
"selectLine:",
"copy:",
"moveToEndOfLine:",
"insertNewline:",
"paste:",
"deleteBackward:"
);
}
This will create two shor...