SEARCH KEYWORD -- SQL


  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

  Never ever touch a programmer

This is a technical license plate block example. We don't know whether it is working or not, however, the idea is very good. When you drive through some of the junctions, the camera captures the license number and convert it into text with OCR, and then insert them into the database. Therefore, this license plate on the car becomes a SQL injection. This picture tells us -- never trust user input.Author : 陈皓 Source : http://coolshell.cn/articles/6639.html#more-6639...

   Programmer,SQL injection     2012-04-21 01:19:48

  <=> 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

  Solve Hibernate "Too many connections" issue in MySQL

When working with Hibernate and MySQL, sometimes some exceptions will be thrown after sometime. The exception may seem like : java.sql.SQLException: Data source rejected establishment of connection, message from server: "Too many connections" This means there are too many active connections on the MySQL, you can use SHOW STATUS LIKE '%Threads_connected%'; to check the active connections to MySQL. If you want to change the maximum connections allowed to MySQL. You can execute: set global max_co...

   MySQL,Hibernate,Clos     2013-09-04 22:20:49

  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

  Oracle database to execute multiple statements at command line

Oracle database is now a mature and popular RDBMS which is used by lots of enterprises. With its evolution, many utilities and applications are developed around it as well. Now Oracle database also provides cloud support. As a DBA, managing Oracle database is daily routine work and some tools may be used frequently. One of them is sqlplus, the command line tool for executing SQL command. sqlplus is installed while the Oracle database is installed. To start sqlplus, one just needs to open a termi...

   ORACLE,ORACLE DATABASE,SQLPLUS     2016-12-09 20:11:28

  What is blocking and how would you troubleshoot it?

Blocking is a common occurrence in an SQL server context, but if you are new to the world of database management you might not know what this issue entails and perhaps even fear that it is a sign of serious underlying problems. To allay your fears and clear up the mystery, here is a brief overview of blocking and the steps you can take to tackle it. Image Source: Pixabay SQL blocking explained SQL blocking according to SentryOne is an offshoot of the way that concurrent databases operate. Becau...

   SQL,DEADLOCK     2020-09-17 08:50:55

  Fix 'this authentication plugin is not supported' issue while using Go to connect MySQL 8

MySQL 8 has changed its default authentication plugin from mysql_native_password to caching_sha2_password to improve its security. However many third party libraries seem act slowly to catch up with this change. This causes some compatible issues with their connection to MySQL. One of the issues is seen in Go libraries while it's trying to connect to MySQL 8. The specific error has been observed is "this authentication plugin is not supported". The root cause of this issue is that the go-sq...

   MYSQL,GO,MYSQL 8,AUTHENTICATION PLUGIN     2018-07-11 08:55:02

  Simple PHP paging class

Frequently in our web applications, we may have many records in the database to display. In order to imrpove loading speed and efficiency, we may need to display some records at a time, so we need to paginate the records. For example, if we have 1 million book records and when users want to view the book list, it's inefficient to display all the records on the same page, we may need to have some pagination to allow displaying a portion of the records such as 20 records per page. This is a simple...

   PHP,Paging,Class     2012-09-10 20:23:33

  How to optimize MySQL insert statement

For a big data system, one problem is the data access efficiency, one more problem is that the data insertion is very slow. We had a service system, the data loading process would take 4-5 hours. This time consuming operation is risky since if the program is interrupted during the loading process, it might be rerun, this will be troublesome. So it's necessary to improve the insertion efficiency for big data systems. Here we provide two optimization suggestions. 1. Combine multiple insert stateme...

   MySQL,insert,optimization     2012-10-24 22:03:13

RECENT