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Technical Articles

Database

SQL

How many primary keys are there in a single data table of the database?

There can be only one primary key in a single data table of the database. A database primary key refers to a column or a combination of multiple columns, whose value can uniquely identify each row in the table, through which the entity integrity of the table can be enforced; the primary key is mainly used to associate foreign keys with other tables, as well as this Modification and deletion of records.

Jul 06, 2020 pm 03:20 PM

数据库主键

What database language does sql belong to?

SQL is a relational database language. Relational database language is a database language with multiple functions such as data manipulation and data definition. This language has interactive characteristics, can provide users with great convenience, and can use SQL language to improve the work quality and performance of computer application systems. efficiency.

Jul 06, 2020 pm 03:15 PM

sql

What are the applications of database management systems?

Applications of database management systems: 1. Improve the security of data encryption systems; 2. Improve information storage efficiency; 3. Improve data backup and recovery; 4. Enhance multimedia management. The database management system provides unified management and control of the database to ensure the security and integrity of the database.

Jul 04, 2020 am 10:10 AM

数据库管理系统应用

The role of database index

The biggest function of a database index is to speed up queries. It can fundamentally reduce the number of record rows that need to be scanned. A database index is the data structure of the database. Furthermore, the data structure stores all the values of a certain column in a table. , that is to say, the index is created based on a certain column in the data table.

Jul 03, 2020 pm 05:19 PM

数据库

What are the SQL statements to delete fields in a table?

SQL statements to delete fields in the table include: 1. Delete columns without default values [alter table Test drop COLUMN BazaarType]; 2. Delete columns with default values [alter table Test DROP COLUMN BazaarType].

Jul 03, 2020 pm 04:20 PM

sql 删除字段

What is the sql statement to modify the table name?

The SQL statement to modify the table name is the "ALTER TABLE" statement; the basic syntax is: "ALTER TABLE <table name> [modification options]". The "ALTER TABLE" statement is used to modify the design of an existing table; columns can be added, modified, or deleted from an existing table.

Jul 03, 2020 pm 03:59 PM

sql

What are the SQL date format conversion functions?

The sql date format conversion functions are: 1. [Select CONVERT(GETDATE(), 23)] formats the current date into [year-month-day] format; 2. [Select CONVERT(GETDATE(), 8] formats the current time Format it into [hour:minute:second] format.

Jul 03, 2020 am 11:33 AM

sql 日期格式

What are the characteristics of nosql database

The characteristics of nosql database are: 1. Easy to expand; 2. Large data volume and high performance; 3. Flexible data model; 4. High availability. There are many types of NoSQL databases, but they all have a common feature, that is, they all remove the relational characteristics of relational databases.

Jul 03, 2020 am 11:05 AM

nosql 特点

What are the relational operations of the database?

The relational operations of the database include: 1. Selection operation, which selects all tuples that meet given conditions from a relation R; 2. Projection operation, which selects certain specified attributes from all attributes of a relation R; 3. Connection operation, It selects tuples that meet certain conditions between attributes from the generalized Cartesian product of two relationships to form a new relationship.

Jul 03, 2020 am 09:28 AM

数据库关系运算

What is the meaning of group by in sql query

The meaning of group by in SQL query is to combine the total function to group the result set according to one or more columns. Its function is to divide a data set into several small areas through certain rules, and then target several A small area for data processing.

Jul 02, 2020 pm 05:19 PM

sql

What kind of database is hbase?

HBase is a distributed, column-oriented open source database. The technology comes from the Google paper "Bigtable: A distributed storage system for structured data" written by Fay Chang.

Jul 01, 2020 pm 04:30 PM

hbase

What are the methods for sql optimization?

SQL optimization methods: 1. Try to avoid using [select *], useless fields will reduce query efficiency; 2. Avoid using in and not in, you can choose between and exists instead; 3. Avoid using or, you can choose union instead.

Jul 01, 2020 pm 03:55 PM

sql 优化

What are non-relational databases?

Non-relational databases include: 1. MongoDB, which is a document-oriented open source NoSQL database; 2. Cassandra, which was developed by Facebook for inbox search; 3. Redis, which is the most famous key-value store.

Jul 01, 2020 pm 03:24 PM

非关系型数据库

What are distributed databases?

Distributed databases include: 1. In the direction of the Internet of Things, time series database products meet the collection, storage and statistics of IoT data, such as InfluxDB, Kudu, kdb, and OpenTSDB; 2. In the direction of transaction relationships, Ant Financial Oceanbase and Tencent TDSQL.

Jul 01, 2020 pm 03:14 PM

分布式数据库