Pitfalls encountered in Mysql
1. Erdog, when you create a table, do you always set Null as the default value?
Damn, how do you know, how simple Null is, how convenient it is to set Null without passing a value, and it doesn’t take up much space, and you don’t need to fill in the code, and you don’t need to judge.
Posted by onion2k on Sun, 20 Nov 2022 01:41:10 +0300
Hello everyone, we meet again, I am your friend Quanzhangjun.
(This is just my little brother's study notes, not a tutorial, if there are any mistakes, please point them out)In cocos creator, by calling WeChat The interface to obtain user authorization information, and then the cloud function saves the user data to the cloud database, ...
Posted by davex on Thu, 17 Nov 2022 09:59:02 +0300
I haven't done this for a long time. By the way, I was asked about it during the interview
Remove duplicate items
select distinct Column name from Table name;
give an example
Before use:(Here you can see that 3 repeat)
select client ID from Orders;
After use:(It can be seen here that onl ...
Posted by dlebowski on Thu, 25 Aug 2022 12:39:11 +0300
The system uses python language for program design. The main contents of the design can be summarized as follows: the teacher-student integrated student information management system, first of all, the administrator (teacher) adds, deletes, modifies, finds and exports student information (in the form of excel form), and the ...
Posted by narch31 on Fri, 01 Jul 2022 21:35:10 +0300
This note is the learning content of Alibaba Cloud Tianchi Dragon Ball Project SQL Training Camp
The link is: https://tianchi.aliyun.com/specials/promotion/aicampsql
Practice Questions - Part 1
Create a view that satisfies the following three conditions (the view name is ViewPractice5_1). Using the product table as a referen ...
Posted by [e]r!k on Wed, 25 May 2022 06:53:14 +0300
Spark SQL is a module that Spark uses to process structured data. It provides a programming abstraction called DataFrame and acts as a distributed SQL query engine. In Hive, Hive SQL is converted into MapReduce and then submitted to the cluster for execution, which greatly simplifies the complexity of writing MapReduce programs, bec ...
Posted by SoaringUSAEagle on Mon, 23 May 2022 18:39:54 +0300
1. count(1) and count(*)
When the amount of data in the table is larger, after analyzing the table, it takes more time to use count(1) than to use count(*)! From the execution plan, count(1) and count(*) have the same effect. However, after the table is analyzed, count(1) will take less time than count(*) (the amount of data within 1w), but it ...
Posted by newbreed65 on Sat, 21 May 2022 19:49:21 +0300
Introduction: This paper will mainly introduce the process and related parameters of reading and writing Clickhouse by FlinkX. The core content will focus on the following three questions: 1 Which version does FlinkX Clickhouse support 2. What are the parameters for Clickhouse to read and write Clickhouse 3. What are the instructions for Clickh ...
Posted by pedroz on Fri, 20 May 2022 16:16:32 +0300
This article describes what a SQL INNER JOIN join is, why it is used, and how to write a SELECT statement that uses a join. It also covers some of the basics of relational database design, including the most commonly used join, the equi-join (also known as the inner join).
One of the most powerful features of SQL is the ability t ...
Posted by shack on Tue, 17 May 2022 04:26:18 +0300
Query statement select
[LIMIT N][ OFFSET M]
select is a query statement, followed by commas to separate multiple columns, and * indicates all columns
Which form is it from after from
where add conditional restrictions
Limit limit the number of displays
Offset indic ...
Posted by Sgarissta on Mon, 16 May 2022 03:42:58 +0300