• On why MySQL does not recommend sub query and join


    Paging query: 1. For MySQL, it is not recommended to use sub query and join because the efficiency of join is hard. Once the amount of data is large, it is difficult to ensure the efficiency. It is strongly recommended to fetch data according to the index table respectively, and then do join and merge […]

  • Union intersection join (inner connection, outer connection) of SQL statements



  • VBS tutorial: function join function


    Join function Returns a string created by a join of many substrings contained in an array. Join(list[,delimiter]) parameter list Required. A one-dimensional array containing substrings to join. Delimiter Optional. The character used to separate substrings in the return string. If omitted, the null character (“”) will be used. IfdelimiterIf it is a zero length string, […]

  • Join buffer principle of MySQL


    1、 MySQL join buffer In the process of MySQL’s join operation, join buffer is an important concept and an important optimization means of MySQL for table join. Although the implementation of this concept is not complicated, it is an important method to realize MySQL join connection optimization, which can greatly improve the efficiency of join […]

  • Detailed explanation of MySQL’s method of using join to optimize SQL


    0. Prepare related tables for the next test For related table building statements, see: https://github.com/YangBaohust/my_ sql Table 1, meridian group +—-+———–+—————–+———————————+ | id | user_name | comment | mobile | +—-+———–+—————–+———————————+ |1 | Buddhist monk of Tang Dynasty | 138245623021-382349| |2 | Sun Wukong | Dou conquers Buddha | 159384292022-483432, + 86-392432| |3 | Zhu […]

  • Several operation code examples of join() function in Python


    This article mainly introduces a variety of Python join() function operation code examples, the article through the example code is very detailed, for everyone’s study or work has a certain reference learning value, need friends can refer to There are. Join() and. Join() in Python os.path.join () two functions, the specific functions are as follows: […]

  • SQL server uses join all to optimize or query speed


    For example: Master, test indicates that the user is the subordinate code of test. After test logs in, you can see the business under the name of test and the business of all subordinate agents. The structure of the related tables is as follows: The user table has about 100000 records |-uid-|-user-|—-site——| | 1 | […]

  • Hash join: a new feature of MySQL 8.0


    MySQL development team officially released MySQL 8.0.18 GA on October 14, 2019, bringing some new features and enhanced functions. One of the most striking is that the multi table join query supports hash join. Let’s first look at the official description: MySQL implements hash join for inner join query. For example, starting from MySQL 8.0.18, […]

  • MySQL 8.0.18 hash join test (recommended)


    Hash Join Hash join does not require any index to execute and is more efficient than the current block nested loop algorithm in most cases. The following is an example code to introduce MySQL 8.0.18 hash join test. The specific contents are as follows: CREATE TABLE COLUMNS_hj as select * from information_schema.`COLUMNS`; INSERT INTO COLUMNS […]

  • MySQL 8.0.18 hash join does not support left / right join connection


    In MySQL 8.0.18, a new function of hash join has been added, which is applicable to the fields that have not been indexed to do equivalent Association query. In previous versions, if the joined fields were not indexed, the query speed would be very slow, and the optimizer would use BNL (block nesting) algorithm. Hash […]

  • An example analysis of the usage of join() in JS array method


    The example of this article describes the usage of join () of JS array method. To share with you for your reference, as follows: Join () method Definition and usage:The join () method is used to put all elements in an array into a string.Elements are separated by the specified delimiter. Syntax: arrayobject. Join (separator) […]

  • Different test methods of several join in SQL Server


    It mainly introduces the differences among inner join, full out join, cross join, left join and right join. Inner join: filter records on both sidesFull out join: both sides are filtered out. The matches that can be matched are listed with nullCross join: list all combinations on both sides, also known as Cartesian set a […]