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2023-01-23   ES  

 SQL Language Introduction  

1.1 SQL Introduction

The full name of
SQL is the “Structudured Query Language”. The earliest IBM St. Joseph Research Lab was developed by System R for its relationship database management system System R. Its predecessor was Square language. The SQL language structure is simple, powerful, and easy to learn, so since IBM was launched in 1981, the SQL language has been widely used. At present, the SQL language has been identified as the international standard of the relationship database system, and is adopted by most commercial relational relationship database systems, such as Oracle, Sybase, DB2, Informix, SQL Server and other database management systems support SQL language as query language.
Structural query language SQL is a language between the relationship between the relationship between the relationship between the relationship between the relationship. Its functions include query, manipulation, definition, and control. It is a general functional database standard language. There is no need to tell SQL how to access the database in the SQL language, as long as the SQL needs what the database is needed.
    Note:can read “SQL” as “Sequel”, or you can read S -Q -L according to the pronunciation of a single letter. The two pronunciations are correct. Each pronunciation has a large number of supporters. In this experiment, it is believed that “SQL” is read as “Sequel”.

1.2 SQL development process

SQL language was proposed in 1974. Due to its rich functions, flexible usage, simple language and easy learning, it is popular among the computer industry and computer users. In October 1986, the Database Commission of the National Agency (ANSI) approved the US standard for SQL as a database language. In June 1987, the International Standardization Organization (ISO) adopted it as an international standard. This standard is also called “SQL86”. With the continuous development of SQL standardization, “SQL89”, “SQL2” (1992) and “SQL3” (1993) appeared one after another. After SQL became international standards, it also had a great impact on the fields outside the database. Many software products combined the data query function of SQL language with graphic function, software engineering tools, software development tools, and artificial intelligence programs.

1.3 SQL database architecture

The architecture of
SQL database is basically a third -level model. SQL terminology is different from traditional relationships. In SQL, the external mode corresponds to the view, the mode corresponds to the basic table, the tuple is called “line”, and the attribute is called “column”. The internal mode corresponds to the storage file.
    SQL database architecture characteristics:
    A SQL mode (SCHEMA) is a collection of tables and constraints.
    A table is a collection of rows, and each row is the sequence of column (column), and each column corresponds to one data item.
    — a watch can be a basic table or a view. The basic table is the table actually stored in the database. The view is a table exported from the basic table or other views. It itself does not store it independently in the database, which means that the data that only stores the view in the database does not store the view of the view. The view is a virtual table.
    A basic table can span one or more storage files, one storage file can also store one or more basic tables, one table can bring several indexes, and the index is also stored in the storage file. Each storage file corresponds to a physical file on an external memory. The logical structure of the storage file constitutes the internal mode of the relationship database.
    Users can use SQL statements to inquire about views and basic tables. From the user’s view, the view and the basic table are the same, both (that is, the form).
    SQL users can be applications or end users. The SQL statement can be embedded in the programs of the host language; the SQL language can also be used as an independent user interface for end users in the interactive environment.

1.4 SQL composition
    SQL is mainly divided into four parts:
    Data definition: This part is also called “DDL“, used to define SQL mode, basic tables, views, and indexes.
    Data manipulation: This part is also called “DML“, data manipulation is divided into two types: data query and data update. Among them, the data update is divided into three operations: inserting, deleting and modifying.
    Data control: This part is also called “DCL“. Data control includes authorization of basic tables and views, descriptions of integrity rules, transaction control statements, etc.
    Embedded SQL use: This part involves the use rules embedded in the SQL statement embedded in the host language program.

1.5 SQL advantage
SQL is widely used to explain its advantages, so that all users, including apps, have benefited from applicants, DBA administrators and end users.
    Non -processed language
SQL is a non -processing language because it processes a record at a time to provide automatic navigation for data. SQL allows users to work on high -level data structure without operating a single record, which can operate record sets. All SQL statements accept sets as inputs and return sets as output. The set feature of SQL allows the result of a SQL statement as the input of another SQL statement. SQL does not require users to specify the method of storage of data.
    Unified language
SQL can be used for the DB activity model of all users, including system administrators, database administrators, apps, decision support system personnel and many other types of end users. The basic SQL command only needs to be able to learn in just a few days, and the advanced command can be mastered within a few days. The previous database management system provided separate languages for the above -mentioned operations, and SQL unified all tasks in one language.
    is a public language for all relationship databases
Since all the main relationship database management system supports SQL language, users can transfer SQL skills from one RDBMS to another. All programs written in SQL can be transplanted.


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