[MySQL basic] general syntax 1

Database related concepts

• database ：

A warehouse for storing data , Data is organized and stored .DataBase（DB）

• Database management system ：

Large software for manipulating and managing databases .DataBase Management System（DBMS）

• SQL：

Programming language for operating relational databases , A set of unified standards for operational relational database are defined .

mysql Data model

1. Relational database （RDBMS）：

• Concept ：

Based on the relational model , A database consisting of multiple interconnected two-dimensional tables .

• Two-dimensional table ：

A table of rows and columns （ Include header 、 That's ok 、 Column , You can also associate a column of data in another table through one column ）.

• A database that stores data based on two-dimensional tables becomes a relational database ; Not a database that stores data based on two-dimensional tables , It's a non relational database .
• characteristic ：

（1） Use tables to store data , use the same pattern , Easy to maintain ;
（2） Use SQL Language operation , Unified standards , Easy to use .

2. Data model ： database 、 surface

• adopt mysql Client connection database management system DBMS, And then through DBMS Operating the database ;
• Use SQL sentence , adopt DBMS Operating the database , And operate the table structure and data in the database ;
• Multiple databases can be created in one database server , A database can also contain multiple tables , A table can contain multiple rows of records .

General grammar and classification ：

• SQL General grammar ：

• SQL Statements can be written in one or more lines , It ends with a semicolon ;
• SQL Statements can use spaces / Indent To enhance the readability of statements ;
• mysql Database SQL Statement is case insensitive , It is recommended to use uppercase ;

• notes ：

（1） Single-line comments ：

– The comment or # Knowledge content

（2） Multiline comment ：

/* The comment */

• SQL classification ：

According to the function , There are four main categories ：DDL、DML、DQL、DCL

• DDL：Data Definition Language

（1） Concept ：

Data definition language , Used to define database objects （ database 、 surface 、 Field ）

（2） Database operation ：

• Query all databases ：

SHOW DATABASES;

• Querying the current database ：

SELECT DATABSE();

• Create database ：

CREATE DATABASE [IF NOT EXISTS] Database name DEFAULT CHARSET ‘ Character set ’ COLLATE ‘ Sort rule ’

• Delete database ：

DROP DATABASE IF EXISTS Database name ;

• Using a database ：

USE Database name ;

（3） Table operations ：

• Table operations - Query creation
• Query all tables in the current database ：

SHOW TABLES;

• View the specified table structure ：

DESC Table name ;

• Query the table creation statement of the specified table ：

SHOW CREATE TABLE Table name ;

• Create a table structure ：

CREATE TABLE Table name (
Field 1 Field 1 type {COMMENT Field 1 notes },
Field 2 Field 2 type {COMMENT Field 2 notes },
…){COMMENT Table annotation };

• Table operations - data type
• value type ：
  
• String type ：
  
• Date time type ：
  
• Table operations - modify
• Add fields ：

ALTER TABLE Table name ADD Field name type ( length ) COMMENT ‘ notes ’ constraint ;

• Change data type ：

ALTER TABLE Table name MODIFY Field name New data types ( length );

• Modify the field name and field type ：

ALTER TABLE Table name CHANGE Old field name new field name type ( length ) COMMENT ‘ notes ’ constraint ;

• Delete field ：

ALTER TABLE Table name DROP Field name ;

• Modify the name of the table ：

ALTER TABLE Table name RENAME TO The new name of the table ;

• Table operations - Delete
• Delete table :

DROP TABLE [IF EXISTS] Table name ;

• Delete the specified table , And recreate the table :( Delete table , The data in the table will also be deleted )

TRUNCATE TABLE Table name ;

• DML：Data Manipulation Language

（1） Concept ：

Data operation language , Used to add, delete, and modify data in database tables

（2） Add data (INSERT)

• Add data to the specified field ：
  INSERT INTO Table name ( Field name 1, Field name 2, …) VALUES ( value 1, value 2, …);

• Add data to all fields ：
  INSERT INTO Table name VALUES ( value 1, value 2, …);

• Batch add data ：
  INSERT INTO Table name ( Field name 1, Field name 2, …) VALUES ( value 1, value 2, …),( value 1, value 2, …),( value 1, value 2, …);
  INSERT INTO Table name VALUES ( value 1, value 2, …),( value 1, value 2, …),( value 1, value 2, …);

matters needing attention ：
A、 When inserting data , The specified field order needs to be the same as the order of values A corresponding .
B、 String and date data should be enclosed in quotation marks ;
C、 The size of the inserted data , It should be within the specified range of the field .

（3） Modifying data (UPDATE)

UPDATE Table name SET Field name 1 = value 1 , Field name 2 = value 2 , … WHERE Conditions ;

matters needing attention ：

• The conditions for modifying the statement can be , There can be no . If there is no condition , All data of the whole table will be modified .

（4） Delete data (DELETE)

DELETE FROM Table name WHERE Conditions ;

matters needing attention ：

• Conditions can be , There can be no . If there is no condition , Delete the data of the entire table .
• You cannot delete the value of a field , have access to update Set the field value to null.

• DQL：Data Query Language

（1） Concept ：

Data query language , A record used to query data in a database table .

（2） sentence ：

SELECT Field list FROM Table name WHERE List of conditions GROUP BY Group field list HAVING Condition list after grouping ORDER BY Sort field list LIMIT Paging parameters ;

（3） Split ：

The basic query ： Without any conditions ;
Conditions of the query ：where;
Aggregate query ：count、max、min、avg、sum;
Group query ：group by;
Sort query ：order by;
Pagination ：limit.

（4） Basic query ：

A、 Query multiple fields ：

• select Field 1, Field 2, Field 3…from Table name ;
• select * from Table name ;

B、 Field set alias ：

• select Field 1 as Alias 1, Field 2 as Alias 2,…from Table name ;
• select Field 1 Alias 1, Field 2 Alias 2,…from Table name ;

C、 Remove duplicate records ：

• select distinct Field list from Table name ;

（5） Conditions of the query ：

A、 grammar ：

• SELECT Field list FROM Table name WHERE List of conditions ;

B、 Conditions ： Fractional comparison operator 、 Logical operators

① Comparison operator ：

② Logical operators ：

（6） Aggregate functions

A、 Common aggregate functions ：

B、 grammar ：SELECT Aggregate functions ( Field list ) FROM Table name ;

matters needing attention ：

• NULL Value is not involved in all aggregate function operations .
• about count Aggregate functions , Count the total number of qualified records , You can also use count( Numbers / character string ) Statistical query in the form of .

（7） Group query ：

A、 grammar ：

• SELECT Field list FROM Table name WHERE Conditions GROUP BY Group field name HAVING Filter conditions after grouping ;

B、where And having The difference between ：

• The timing of execution is different ：
  where Is to filter before grouping , dissatisfaction where Conditions , Don't participate in grouping ;
  having Is to filter the results after grouping .
• Different judgment conditions ：
  where Aggregate functions cannot be judged ;
  having You can judge the aggregation function .

C、 matters needing attention ：

• After grouping , The query fields are generally aggregate functions and grouping fields , The query field has no meaning .
• Execution order ：where > Aggregate functions > having
• Support multi field grouping , Specific syntax ：group by Field 1, Field 2

（8） Sort query ：

A、 grammar ：

• SELECT Field list FROM Table name ORDER BY Field 1 sort order 1, Field 2 sort order 2;

B、 sort order ：

• ASC： Ascending （ The default value is ）
• DSC： Descending

C、 matters needing attention ：

• If it's in ascending order , You may not specify the sorting method as ASC;
• If it is multi field sorting , When the first field is the same , Will be sorted according to the second field .

（9） Paging query ：

A、 grammar ：

• SELECT Field list FROM Table name LIMIT Starting index , Number of query records ;

B、 matters needing attention ：

• Starting index from 0 Start , Starting index = ( Look up the page number - 1) * Each page shows the number of records ;
• If the query is the first page of data , The starting index can be omitted , Directly abbreviated as limit Number of query records ;

（10） Execution order ：

A、 Writing order ：

B、 Execution order ：

• DCL：Data Control Language

（1） Concept ：

Data control language , Used to manage database users 、 Control access to the database .

（2） Manage users ：

Query the user ：

• SELECT * FROM MYSQL.UAER;

Create user ：

• CREATE USER ‘ user name ’@‘ Host name ’ IDENTIFIED BY ‘ password ’;

Change user password ：

• ALTER USER ‘ user name ’@‘ Host name ’ IDENTIFIED WITH MYSQL_NATIVE_PASSWORD BY ‘ New password ’;

Delete user ：

• DROP USER ‘ user name ’@‘ Host name ’ ;

matters needing attention ：

• stay MySQL The user name is required in @ Host name method , To uniquely identify a user ;
• Host name can be used % Pass through .

（3） Access control ：

• Common permissions ：
  

Query authority ：

• SHOW GRANTS FOR ‘ user name ’@‘ Host name ’;

Grant authority ：

• SHOW GRANTS FOR ‘ user name ’@‘ Host name ’;

Revoke authority ：

• REVOKE Permission list ON Database name . Table name FROM’ user name ’@‘ Host name ’;

matters needing attention ：

• Between multiple permissions , Separated by commas ;
• Authorization time , Database name and table name can use * Carry out general matching , On behalf of all .