SEARCH KEYWORD -- Correlated query


  Select top 3 values from each group in a table with SQL

Yesterday, my friend Liu Bing asked me a question about how to select top 3 values for each person in a database table using SQL. I think this question is interesting and it deserves some thoughts. Here I record down how to solve this issue. Assume we have a table which has two columns, one column contains the names of some people and the other column contains some values related to each person. One person can have more than one value. Each value has a numeric type. The question is we want to se...

   SQL,Correlated query,top 3     2013-05-23 03:21:25

  Why MySQL 8 drops support of query cache

Many of you may have heard or used MySQL's query cache, because it used to be a popular way to improve MySQL's performance. As an important feature for improving MySQL's performance, the query cache was often recommended as a solution for slow queries. However, why has MySQL 8 abandoned the query cache? Today, we will analyze and explore this decision. What is query cache? According to official document: The query cache stores the text of a SELECT statement together with the correspon...

   MYSQL 8,QUERY CACHE     2023-03-11 09:05:17

  Different ways to pass query parameters in EmberJS

In EmberJS, one could pass query parameters when retrieving resources with store.query() method. But what if there is a requirement that one wants to pass query parameters when calling store.findRecord()? Or there is a requirement that one wants to pass query parameters to a relationship when calling model.get('hasManyAttribute') in a RESTful style? In this post, we will explain how these can be achieved. In the store.query() case, one could easily pass the query parameters by passing ...

   RELATIONSHIP,EMBERJS,QUERY PARAMETERS     2018-05-18 10:47:56

  strange thing in PHP session variable and local variable

A few minutes ago, I noticed one strange thing when I did my PHP web application development.The situation is described below:I want to display a user's profile using a query parameter user=user_id as the parameter. And also the session variable when user logs in  is $_SESSION["user"].So before displaying the user's profile, I need to either get the query string parameter user and create a local variable called $user and assign value to it as $user=$_REQUEST["user"].Here comes the strange t...

   PHP,session,variable name,change automat     2011-06-13 12:23:59

  One Way to Quickly Locate SQL with High CPU Usage in MySQL

When the CPU usage of a MySQL database rises abnormally, it's necessary to quickly identify the problematic SQL statements. In this post, we try to provide one way to achieve this.  Below are the actual steps. 1. Use the top command to find the threads with the highest CPU usage in the MySQL process. # Find the MySQL process ID ps -ef | grep mysql # Use the process ID to find the thread IDs with the highest CPU usage: top -H -p In top, press P to sort by CPU usage. Note the thread ID, fo...

   MYSQL,DEBUG,HIGH CPU,SLOW QUERY     2024-08-01 21:25:48

  Fuzzy search algorithm in Sublime Text

Fuzzy string searching is the technique of finding strings that match a pattern approximately (rather than exactly. In many editors, we can easily find the Find menu item to do exact matching. While in Sublime Text and some other editors, we can use fuzzy string searching as well if we just want to have an approximate match. There is some algorithm implemented in Sublime Text to implement achieve this. First the search query is split into characters, join them with a regex wildcard, and then run...

   Sublime Text,Fuzzy search     2013-10-14 22:49:38

  A trap in PDOStatement::bindParam

First, let's check out below codes: <?php $dbh = new PDO('mysql:host=localhost;dbname=test', "test"); $query = <<prepare($query); $bind_params = array(':username' => "laruence", ':password' => "weibo"); foreach( $bind_params as $key => $value ){ $statement->bindParam($key, $value); } $statement->execute(); What is the SQL executed finally? Is there any problem with above codes? Many people may think the query executed is : INSERT INTO `user` (`username`, `password...

   PHP,Trap,bindParam     2013-08-29 10:48:55

  Hologres vs AWS Redshift

Hologres and Redshift are both data warehousing solutions, but they have some differences in terms of features, architecture, and target use cases. Underlying Infrastructure Hologres: Built on Alibaba Cloud's Apsara distributed computing platform, Hologres leverages the underlying infrastructure for storage, computation, and management. It benefits from Alibaba's expertise in cloud-native architecture and real-time data processing. Redshift: Amazon Redshift is based on a Massively Parallel Pro...

   HOLOGRES,REDSHIFT,ALIBABA,AWS,BIG DATA,REAL-TIME     2024-03-23 01:36:41

  <=> operator in MySQL

Have you ever seen "<=>" in a SQL query while using MySQL? Does it mean less and equals to and greater than? Actually if you consider it as the union of <= and =>, great, you are close to it. This is one form of equal operator in MySQL, it has the similar meaning to the = operator with some subtle difference. According to MySQL documentation, <=> is NULL-safe equal. This operator performs an equality comparison like the = operator, but returns 1 rather than NULL if both operand...

   MySQL,NULL safe,<=>     2014-03-24 06:23:22

  OpenLDAP Proxy -- Introduction

OpenLDAP is an open source project which is intended to provide some commercial-grade, fully featured applications and development tools based on the well known Light-weight Directory Access Protocol(LDAP). Hence corporations which want to manage their user and group information can freely use these tools.  OpenLDAP provides ldap servers, ldap clients and corresponding tools to work on LDAP. The LDAP server is called slapd(Stand-alone LDAP  daemon). And it would serve as the server whi...

   META,OPENLDAP,OPENLDAP PROXY,SLAPD,SLAPD.CONF,LDAP     2017-10-28 11:20:00

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