大约有 40,000 项符合查询结果（耗时：0.0450秒） [XML]
Why is SELECT * considered harmful?
...concept that you're counting rows, and you avoid strange edge-cases caused by NULLs being eliminated from your aggregates.
Same goes with this type of query:
SELECT a.ID FROM TableA a
WHERE EXISTS (
SELECT *
FROM TableB b
WHERE b.ID = a.B_ID);
in any database worth its salt, * just...
Remove duplicated rows using dplyr
...oach would be to group, and then only keep the first row:
df %>% group_by(x, y) %>% filter(row_number(z) == 1)
## Source: local data frame [3 x 3]
## Groups: x, y
##
## x y z
## 1 0 1 1
## 2 1 0 2
## 3 1 1 4
(In dplyr 0.2 you won't need the dummy z variable and will just be
able to writ...
JOIN queries vs multiple queries
...ark" might have some merit. But left or inner joins are the norm, usually by key (making retrieval much faster), and the duplication of data is usually much, much less than you're making it out to be.
– cHao
May 3 '11 at 22:25
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How to reshape data from long to wide format
...
I would say base R still wins vote-wise by a factor of about 2 to 1
– vonjd
Nov 22 '18 at 15:14
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show 1 ...
Warn user before leaving web page with unsaved changes
...
Short, wrong answer:
You can do this by handling the beforeunload event and returning a non-null string:
window.addEventListener("beforeunload", function (e) {
var confirmationMessage = 'It looks like you have been editing something. '
...
2 column div layout: right column with fixed width, left fluid
... before the left, simply use a negative right-margin.
And be "responsive" by including an @media setting so the right column falls under the left on narrow screens.
<div style="background: #f1f2ea;">
<div id="container">
<div id="content">
<strong>Column 1 - c...
Why Does OAuth v2 Have Both Access and Refresh Tokens?
...
Refresh token can be used by a third party that can renew the access token without any knowledge of user credentials.
– Marek Dec
Dec 22 '15 at 15:58
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SQL - find records from one table which don't exist in another
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An advantage of this approach (vs. LEFT OUTER JOIN by WOPR) is that it avoids returning multiple rows per row of Call, if there are multiple matching rows in Phone_book. That is, if there is a 1:N relationship between your two tables.
– ToolmakerSteve
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Update Angular model after setting input value with jQuery
... This solved a major issue when using jquery calendar plugins. By hacking the plugin to trigger('input') after input.val() the ng-change event fires after user selects calendar date. Thanks!
– Drone Brain
Oct 3 '13 at 20:07
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How to create a GUID/UUID using iOS
...i use [[NSUUID UUID] UUIDString]; in iOS7 and above, won't my app rejected by Apple?
– Whoami
Sep 9 '14 at 7:25
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