General syntax and classification of SQL language (II)

SQL General grammar and classification of languages

1.SQL The general grammatical format of language

No matter what kind of database products ,SQL The grammar is universal .

• SQL Statements can be written in a single line or in multiple lines , It ends with a semicolon .
• SQL Statements can use spaces or indents to enhance the readability of statements , There is no limit to the number of spaces and indents .
• MySQL Database SQL Statements are case insensitive , It is recommended to use uppercase for keywords , Relatively standardized , Of course, it also depends on personal habits .
• There are two cases about annotation ：
  • Single-line comments ：– Comment content or # The comment （# Note number is MySQL A unique way of writing ）
  • Multiline comment ：/* The comment */

2.SQL Several classifications of language

1）DDL type

DDL Type of SQL The full name of the language is Data Definition Language, The Chinese name is data definition language , It is mainly used to define objects in the database , Like databases 、 Definition of tables and fields , The simple understanding is DDL Language is to operate database 、 Of tables and fields .

2）DML type

DML Type of SQL The full name of the language is Data Manipulation Language, The Chinese name is data manipulation language , It is mainly used to add data in the database 、 Delete 、 Change the operation , For example, add a piece of data 、 Delete a piece of data 、 Modify a piece of data and so on .

3）DQL type

DQL Type of SQL The full name of the language is Data Query Language, The Chinese name is data query language , It is mainly used to query the data of tables in the database , Like association query 、 Multi-table query 、 Embedded