Related explanations about left join on and where condition placement

The meaning of JOIN is just like the English word "join". It joins two tables and can be roughly divided into inner join, outer join, right join, left join and natural join. First create two tables, the following is used as an example CREATETABLEt_blog(idINTPRIMARYKEYAUTO_INCREMENT,titleVARCHAR(50),typeIdINT);SELECT*FROMt_blog;+----+------+--------+| id|title|typeId|+----+-------+--------+|1|aaa|1||2|bbb|2||3|ccc|3|

The left keyboard is the left direction key, and the right is the right direction key. Generally, the keyboard is replaced by symbols or arrows. Some keyboards are labeled in English. The keyboard is an instruction and data input device used to operate the equipment, and also refers to the system arrangement. A set of function keys that operate a machine or piece of equipment.

Join type leftjoin uses the left table as the driving table and the left table as the basis of the result set. The data from the right table is connected to the result set. rightjoin uses the right table as the driving table and the right table as the basis of the result set to connect the left table. The data is added to the result set innerjoin. The result set takes the intersection of the two tables fulljoin. The result set takes the union of the two tables. MySQL does not have a fulljoin. The difference between union and unionall is that union will deduplicate the crossjoin Cartesian product. If the where condition is not used, the result set will be the product and of the two associated table rows. The difference is that when crossjoin creates the result set, it will be passed according to the on condition.

Practical Guide to Where Method in Laravel Collections During the development of the Laravel framework, collections are a very useful data structure that provide rich methods to manipulate data. Among them, the Where method is a commonly used filtering method that can filter elements in a collection based on specified conditions. This article will introduce the use of the Where method in Laravel collections and demonstrate its usage through specific code examples. 1. Basic usage of Where method

Introduction A's unique + AB's public B's unique + AB's public AB's public A's unique B's unique A's unique + B's unique + AB's public A's unique + B's unique Practice creating table department tables DROPTABLEIFEXISTS`dept`;CREATETABLE`dept`(`dept_id`int(11)NOTNULLAUTO_INCREMENT,`dept_name`varchar(30)DEFAULTNULL,`dept_number`int(11)DEFAULTNULL,PRIMARYKEY(`dept_id`))ENGINE =InnoDBAUT

Compared with join query and multiple queries, which one is more efficient, MySQL multi-table related query or multiple single-table query? When the amount of data is not large enough, there is no problem using join, but it is usually done on the service layer. First: the computing resources of a stand-alone database are very expensive, and the database needs to serve both writing and reading at the same time, which requires CPU consumption. In order to make the database The throughput becomes higher, and the business does not care about the delay gap of hundreds of microseconds to milliseconds. The business will put more calculations into the service layer. After all, computing resources can be easily expanded horizontally, and databases are difficult, so most The business will put pure computing operations into the service layer, and use the database as a kv system with transaction capabilities. This is a heavy business.

How to use the Where method in Laravel collection Laravel is a popular PHP framework that provides a wealth of functions and tools to facilitate developers to quickly build applications. Among them, Collection is a very practical and powerful data structure in Laravel. Developers can use collections to perform various operations on data, such as filtering, mapping, sorting, etc. In collections, the Where method is a commonly used method for filtering the collection based on specified conditions.

From beginner to proficient: Master the skills of using is and where selectors Introduction: In the process of data processing and analysis, the selector is a very important tool. Through selectors, we can extract the required data from the data set according to specific conditions. This article will introduce the usage skills of is and where selectors to help readers quickly master the powerful functions of these two selectors. 1. Use of the is selector The is selector is a basic selector that allows us to select the data set based on given conditions.