Getting Started with SQL Databases (Basic Concepts and Operations Guide for Getting Started with SQL Databases)
An SQL database is a relational database that uses the SQL language to manage and manipulate data.SQL is an acronym for StructuredQueryLanguage, which is a standardized language used to manage relational databases.SQL databases are widely used in the business, government, and personal fields and are a very popular type of database.
Basic Concepts of SQL Databases
Tables
Data in SQL databases are organized into tables, each consisting of multiple rows and columns. Each row in a table represents a record and each column represents an attribute. For example, a student table might include attributes such as the student’s name, age, and gender.
Primary Key
Every table must have a primary key, which is used to uniquely identify each row in the table. The primary key can be a single column or a combination of columns. For example, in the student table, each student’s student number can be used as the primary key.
Foreign key
A foreign key is a column in a table that references a primary key column in another table. With a foreign key, you can establish an association between different tables. For example, a foreign key relationship can be established between the Students table and the Courses table to represent the association between students and courses.
Indexes
An index is a data structure used to speed up queries. It can improve query efficiency, but can increase the time for data insertion, update and deletion. In a SQL database, indexes can be created for columns in a table.
Guide to SQL databases
Creating a database
In SQL databases, you can use the CREATEDATABASE statement to create a new database. For example, to create a database named mydatabase, you can use the following statement:
“`
CREATEDATABASEmydatabase;
“`
CREATING TABLES
Creating a table in a database can be done using the CREATETABLE statement. For example, to create a table named students with the name, age, and gender attributes of the students, the following statement can be used:
“`
CREATETABLEstudents(
idINTPRIMARYKEY,
nameVARCHAR(50) ,
ageINT,
genderVARCHAR(10)
);
“
Inserting Data
To insert data into a table, you can use the INSERTINTO statement. For example, to insert a new record into the students table, you can use the following statement:
“`
INSERTINTOstudents(id,name,age,gender)
VALUES(1,’Zhangsan’,18,’male’);
“`
Querying Data
To query data from a table, you can use the SELECT statement. For example, to query the names and ages of all students in the students table, you can use the following statement:
“
SELECTname,ageFROMstudents;
“
Update data
To update data in a table, you can use the UPDATE statement. For example, to update the age of the student with the middle school number 1 in the students table to 20, you can use the following statement:
“`
UPDATEstudentsSETage=20WHEREid=1;
“`
DELETE DATA
To delete data from the table, you can use the DELETE statement. For example, to delete the student record in thestudents table with a student ID of 1, you can use the following statement:
“`
DELETEFROMstudentsWHEREid=1;
“`
How to set database identifier in sql?
The following four points are necessary to fill in the identifier.
1, identifier consists of letters, numbers, underscore (_) and dollar sign ($), can not start with a number; 2, can not use java keywords and reserved words as identifiers. 3, identifiers have no length limit. 4, identifiers are case sensitive.
The identifier is equivalent to the name of the variable or constant, the name of the database object is its identifier.
Everything in MicrosoftSQLServer can have an identifier. The server, database, and database objects (such as tables, views, columns, indexes, triggers, procedures, constraints, and rules, etc.) can have identifiers, and most objects require identifiers, but for some (such as constraints), identifiers are optional.
Object identifiers are created when an object is defined, and the identifier is then used to refer to the object; for example, the following statement creates a table with the identifier TableX, which has two columns with the identifiers KeyCol and Description.
The ternary of an ipsec is 1, a Security Parameter Index (SPI) . 2, a source or destination IP address. 3, security protocol (AH and ESP) identifiers.
The general format for defining symbolic constants: CONST<Constant identifier>=<Constant>Description: The constant description section starts with the keyword const, followed by the identifier for the constant identifier, where the constant after the “=” sign is an integer, real number, word.
How to have SQL statement to create two primary keys for a table in SQL database?
In the definition of the table, the field only define the data type and length on the line, in the end add constraints on the line, similar: constraint
pk_1
primary
key(*,*)