Something happened today-MySQL, something happened-MySQL

Home

Backend Development

PHP Tutorial

Something happened today-MySQL, something happened-MySQL_PHP tutorial

WBOYWBOYWBOYWBOYWBOYWBOYWBOYWBOYWBOYWBOYWBOYWBOYWB

Jul 12, 2016 am 09:05 AM

mysql

Something happened today-MySQL, something happened-MySQL

I have something to do today, I have to play for a while, I can learn as much as I can

1. MySQL

-----Subquery and Connection (2)-----

----Subquery

Subquery refers to the SELECT clause

that appears in other SQL statements

Note: A subquery is nested inside a query and must always appear within parentheses; it can contain multiple keywords or conditions; the outer query can be SELECT, INSERT, UPDATE, SET or DO

The return of the subquery can be a scalar, a row, a column or a subquery

can be divided into three categories: using comparison operators =, >=, etc.; NOT IN; EXIST

----Use comparison operators to implement subqueries

mysql> SELECT goods_id,goods_name,goods_price FROM tdb_goods WHERE goods_price>=5391.30;

You can ignore the number here, it is an average value obtained previously.

So if you want to directly get the result of the average value, use a subquery to write like this

mysql> SELECT goods_id,goods_name,goods_price FROM tdb_goods WHERE goods_price>=(SELECT ROUND(AVG(goods_price),2) FROM tdb_goods);

Here you can see that the subquery is in parentheses, using >=

When the subquery returns multiple results, but the main query does not use so many results, you can use the ANY SOME ALL keyword

ANY is the same as SOME. Just satisfy one of them. The specific one depends on the operator (for example, > is greater than the minimum value)

ALL requires that all returned requirements be met

Usage is written in front of the subquery parentheses

----Subquery triggered by IN or NOT IN

In fact, IN is equivalent to the =ALL operator, and NOT IN is equivalent to =! ALL equivalent

Usage is also similar

----EXIST

Returns a Boolean value, rarely used

Statement

The content of this article is voluntarily contributed by netizens, and the copyright belongs to the original author. This site does not assume corresponding legal responsibility. If you find any content suspected of plagiarism or infringement, please contact admin@php.cn

How can you check if a PHP session has already started?Apr 30, 2025 am 12:20 AM

In PHP, you can use session_status() or session_id() to check whether the session has started. 1) Use the session_status() function. If PHP_SESSION_ACTIVE is returned, the session has been started. 2) Use the session_id() function, if a non-empty string is returned, the session has been started. Both methods can effectively check the session state, and choosing which method to use depends on the PHP version and personal preferences.

Describe a scenario where using sessions is essential in a web application.Apr 30, 2025 am 12:16 AM

Sessionsarevitalinwebapplications,especiallyfore-commerceplatforms.Theymaintainuserdataacrossrequests,crucialforshoppingcarts,authentication,andpersonalization.InFlask,sessionscanbeimplementedusingsimplecodetomanageuserloginsanddatapersistence.

How can you manage concurrent session access in PHP?Apr 30, 2025 am 12:11 AM

Managing concurrent session access in PHP can be done by the following methods: 1. Use the database to store session data, 2. Use Redis or Memcached, 3. Implement a session locking strategy. These methods help ensure data consistency and improve concurrency performance.

What are the limitations of using PHP sessions?Apr 30, 2025 am 12:04 AM

PHPsessionshaveseverallimitations:1)Storageconstraintscanleadtoperformanceissues;2)Securityvulnerabilitieslikesessionfixationattacksexist;3)Scalabilityischallengingduetoserver-specificstorage;4)Sessionexpirationmanagementcanbeproblematic;5)Datapersis

Explain how load balancing affects session management and how to address it.Apr 29, 2025 am 12:42 AM

Load balancing affects session management, but can be resolved with session replication, session stickiness, and centralized session storage. 1. Session Replication Copy session data between servers. 2. Session stickiness directs user requests to the same server. 3. Centralized session storage uses independent servers such as Redis to store session data to ensure data sharing.

Explain the concept of session locking.Apr 29, 2025 am 12:39 AM

Sessionlockingisatechniqueusedtoensureauser'ssessionremainsexclusivetooneuseratatime.Itiscrucialforpreventingdatacorruptionandsecuritybreachesinmulti-userapplications.Sessionlockingisimplementedusingserver-sidelockingmechanisms,suchasReentrantLockinJ

Are there any alternatives to PHP sessions?Apr 29, 2025 am 12:36 AM

Alternatives to PHP sessions include Cookies, Token-based Authentication, Database-based Sessions, and Redis/Memcached. 1.Cookies manage sessions by storing data on the client, which is simple but low in security. 2.Token-based Authentication uses tokens to verify users, which is highly secure but requires additional logic. 3.Database-basedSessions stores data in the database, which has good scalability but may affect performance. 4. Redis/Memcached uses distributed cache to improve performance and scalability, but requires additional matching

Define the term 'session hijacking' in the context of PHP.Apr 29, 2025 am 12:33 AM

Sessionhijacking refers to an attacker impersonating a user by obtaining the user's sessionID. Prevention methods include: 1) encrypting communication using HTTPS; 2) verifying the source of the sessionID; 3) using a secure sessionID generation algorithm; 4) regularly updating the sessionID.

See all articles