大约有 40,000 项符合查询结果（耗时：0.0217秒） [XML]
What are the mechanics of short string optimization in libc++?
...of(__long) - 1)/sizeof(value_type) : 2};
struct __short
{
union
{
unsigned char __size_;
value_type __lx;
};
value_type __data_[__min_cap];
};
union __ulx{__long __lx; __short __lxx;};
enum {__n_words = sizeof(__ulx) / si...
COUNT(*) vs. COUNT(1) vs. COUNT(pk): which is better? [duplicate]
...for all your queries that need to count everything, even for joins, use *
SELECT boss.boss_id, COUNT(subordinate.*)
FROM boss
LEFT JOIN subordinate on subordinate.boss_id = boss.boss_id
GROUP BY boss.id
But don't use COUNT(*) for LEFT joins, as that will return 1 even if the subordinate table doe...
What is the difference between 127.0.0.1 and localhost
...
/etc/nsswitch.conf selects if hosts or DNS is used first for a host lookup, if nss is running.
– Mark Lakata
Feb 14 '19 at 20:16
...
Can I use a function for a default value in MySql?
... update previously existing rows, like this:
UPDATE app_users SET uuid = (SELECT uuid());
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How do I get the size of a java.sql.ResultSet?
...
Do a SELECT COUNT(*) FROM ... query instead.
OR
int size =0;
if (rs != null)
{
rs.last(); // moves cursor to the last row
size = rs.getRow(); // get row id
}
In either of the case, you won't have to loop over the enti...
When to use SELECT … FOR UPDATE?
Please help me understand the use-case behind SELECT ... FOR UPDATE .
2 Answers
2
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Best database field type for a URL
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The URL really might break the 65,535 byte row limit
Your queries won't select or update a bunch of URLs at once (or very often). This is because TEXT columns just hold a pointer inline, and the random accesses involved in retrieving the referenced data can be painful.
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How can I get column names from a table in SQL Server?
... much more by querying the Information Schema views.
This sample query:
SELECT *
FROM INFORMATION_SCHEMA.COLUMNS
WHERE TABLE_NAME = N'Customers'
Can be made over all these DB objects:
CHECK_CONSTRAINTS
COLUMN_DOMAIN_USAGE
COLUMN_PRIVILEGES
COLUMNS
CONSTRAINT_COLUMN_USAGE
CONSTRAINT_...
Find the number of columns in a table
...
SELECT COUNT(*)
FROM INFORMATION_SCHEMA.COLUMNS
WHERE table_catalog = 'database_name' -- the database
AND table_name = 'table_name'
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PHP date() format when inserting into datetime in MySQL
...FROM_UNIXTIME(1231634282));
Query OK, 1 row affected (0.00 sec)
mysql> select * from a_table;
+---------------------+
| a_date |
+---------------------+
| 2009-01-10 18:38:02 |
+---------------------+
shar...