大约有 37,000 项符合查询结果（耗时：0.0160秒） [XML]
What do Clustered and Non clustered index actually mean?
... all the columns. You do not have to go first to the index and then to the table.
Writing to a table with a clustered index can be slower, if there is a need to rearrange the data.
share
|
improve ...
Postgresql - change the size of a varchar column to lower length
I have a question about the ALTER TABLE command on a really large table (almost 30 millions rows).
One of its columns is a varchar(255) and I would like to resize it to a varchar(40) .
Basically, I would like to change my column by running the following command:
...
Auto increment in phpmyadmin
... column. I guess the phpMyAdmin version is 3.5.5 but not sure.
Click on Table > Structure tab > Under Action
Click Primary (set as primary),
click on Change on the pop-up window, scroll left and check A_I. Also make sure you have selected None for Default
...
How can I copy data from one column to another in the same table?
Is it possible to copy data from column A to column B for all records in a table in SQL?
3 Answers
...
How to add multiple columns to a table in Postgres?
...
Try this :
ALTER TABLE table ADD COLUMN col1 int, ADD COLUMN col2 int;
share
|
improve this answer
|
follow
...
In MySQL what does “Overhead” mean, what is bad about it, and how to fix it?
...uld only worry if this gets really high.
You can compare 'Optimizing the table' with the defragmenting of your hard drive.
I quote:
Every database will, over time,
require some form of maintenance to
keep it at an optimal performance
level. Purging deleted rows,
resequencing, compre...
Nullable Foreign Key bad practice?
Let's say you have a table Orders with a foreign key to a Customer Id. Now, suppose you want to add an Order without a Customer Id, (whether that should be possible is another question) you would have to make the foreign key NULL... Is that bad practice or would you rather work with a link table bet...
When should I use cross apply over inner join?
...readable and probably less efficient.
Update:
Just checked.
master is a table of about 20,000,000 records with a PRIMARY KEY on id.
This query:
WITH q AS
(
SELECT *, ROW_NUMBER() OVER (ORDER BY id) AS rn
FROM master
),
t AS
(
SELEC...
Oracle: how to UPSERT (update or insert into a table?)
The UPSERT operation either updates or inserts a row in a table, depending if the table already has a row that matches the data:
...
Is there a better way to dynamically build an SQL WHERE clause than by using 1=1 at its beginning?
... offer you:
using (var dbContext = new MyDbContext())
{
IQueryable<Table1Item> query = dbContext.Table1;
if (condition1)
{
query = query.Where(c => c.Col1 == 0);
}
if (condition2)
{
query = query.Where(c => c.Col2 == 1);
}
if (condition3)...