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what is json

JSON (JavaScript Object Notation) is a lightweight data exchange format that is easy to read and write, as well as easy to be parsed and generated by machines. .

Jun 10, 2019 pm 01:16 PM

json

What is the order of sql execution?

The SQL statement is divided into 11 steps. The FROM operation is always executed first, and the LIMIT operation is executed last. Each operation will generate a virtual table, which is used as an input for processing. However, these virtual tables are transparent to the user, and the last virtual table will be returned as a result.

Jun 10, 2019 am 10:36 AM

sql执行顺序

What are the commonly used relational databases?

Simply put, a relational database is a database composed of multiple two-dimensional row and column tables that can be connected to each other. The current mainstream relational databases include Oracle, DB2, Microsoft SQL Server, Microsoft Access, MySQL, etc.

Jun 06, 2019 pm 01:11 PM

关系型数据库数据库

Several methods of sql optimization

To optimize queries, you should try to avoid full table scans. You should first consider building indexes on the columns involved in where and order by. You should try to avoid using or in the where clause to connect conditions, otherwise the search engine will give up using the index and Perform a full table scan.

Jun 05, 2019 pm 04:04 PM

sql优化

Database type

Databases are divided into relational databases and non-relational databases. Commonly used relational databases include MySQL, MariaDB database, SQL Server database, and Access database; commonly used non-relational databases include memcached, Redis, MongoDB, Cassandra, etc.

Jun 05, 2019 am 10:41 AM

数据库

What are the differences between delete and truncate

The difference between delete and truncate commands: delete is a data manipulation language (DML) command, which uses row-level locking, and can delete single, multiple or all records from the table; while truncate is a data definition language (DDL) command, which uses table-level locking. , delete all records and table structures from the database. etc.

Jun 03, 2019 pm 03:05 PM

sqldeletetruncate

Why use redis?

Redis is an open source log-type, [Key-Value] database written in ANSI C language, supports network, can be memory-based and persistent, and provides APIs in multiple languages; using redis can greatly improve system access efficiency.

May 21, 2019 pm 02:52 PM

redis

When do you need to use redis?

When you need to use redis: 1. Data that does not need to be updated in real time but consumes a lot of database data; 2. Data that needs to be updated in real time, but the update frequency is not high; 3. The number of visits at a certain moment is very high and the updates are also very frequent. data.

May 21, 2019 pm 02:44 PM

redis

Is the redis database free?

Redis is completely open source and free. It is an open source log-type Key-Value database written in ANSIC language, abiding by the BSD protocol, supporting the network, memory-based and persistent, and providing APIs in multiple languages.

May 21, 2019 am 10:44 AM

redis

How to perform sql injection

Methods for SQL injection: 1. Force an error; 2. Use non-mainstream channel technology; 3. Use special characters; 4. Use conditional statements; 5. Use stored procedures; 6. Avoid input filtering technology, etc.

May 18, 2019 am 10:14 AM

sql

How to recover deleted data from sql database

SQL database data recovery method: 1. Back up the transaction log of the current database [BACKUP LOG [database name] TO disk= N'backup file name' WITH NORECOVERY]; 2. Restore a full backup before accidental deletion; 3. Restore the database Restore to a point in time before accidental deletion.

May 17, 2019 pm 05:55 PM

sql

Is SQL easy to learn?

In fact, everything has two sides. It will become easier as long as you work hard, believe in yourself. There are no shortcuts to learning, you can only rely on hard work, and if you are a programmer learning databases, you don’t need to know much, just know the basic operations. If you specialize in databases, you will need a lot more.

May 17, 2019 pm 04:56 PM

SQLmysql

how to learn sql

How to learn sql: first you need to master certain basic knowledge of database; then you need to find some sql exercises to do; finally you need to learn advanced sql usage and techniques. sql is a database query and programming language used to access data and query, update and manage relational database systems.

May 17, 2019 pm 03:50 PM

sql

[SQL Learning] The connection between ordered index and order by

Under normal circumstances, there are two implementation methods for order by. One is to automatically implement it using an ordered index. That is to say, using the orderliness of the ordered index does not require additional sorting operations. The other is to sort the results after selecting them. Using an ordered index is of course the fastest, but there are some restrictions. Let’s take a look at the test below.

Apr 18, 2019 am 10:43 AM

SQL有序索引order by